"When I come back from New York," Susan Sheehan was saying, "the kids watch me unpack the suitcase to see what food I've brought home. It's cheaper to shop for groceries in New York, so I do. My friend Jane Goldwyn just came back with a carload of groceries."

If this raises visions of homeward-bound Washingtonians boarding the shuttle with grocery bags instead of attache cases, or of an agricultural inspector checking baggage at National Airport, read on.

The jewels and clothing that once attracted shoppers to the Big Apple are now here, in local editions of swank stores such as Bloomingdale's and Saks Fifth Avenue. What isn't available locally, according to Sheehan, is coffee at $2.29 a pound or less; veal on sale for $1.50 a pound less than it costs in our supermarkets; tuna fish at 50 or 55 cents a can (one "on sale" lure here recebtly was 3 cans of tuna for $2) and numerous other items.

The interest in food prices and sale items Sheehan and her friends have developed reflects a growing pattern among even upper-middle-class consumers as inflation continues to shrink disposable income (the share of total income that isn't precommitted to items such as mortgage or installment payments.)

"My shopping pattern has changed," said Sheehan. "My head is cluttered with prices I just as soon weren't there. I used to look at the ads and buy what I needed on a weekly basis. Now if something, say paper towels, is on sale, I'll buy 20. When frozen strawberries were at 99 cents instead of $1.19, I bought 40 bags.

"Some friends are raising cattle. We have a cabin in West Virginia and when we're up there we buy rabbits from a neighbor farm boy. Everybody's waging his own war. there's a real sense of accomplishment when I save $15 or $20, although I probably waste it elsewhere. But by finding alternatives, you don't have a complete sense of helplessness."

Jane Goldwyn, who said not so long ago she "never read ads" but "bought what I had (wanted) to buy, whether it was on sale of not," laughed as she recoounted loading her compact station wagon in New York. There were, among other items, coffee, large cans of tomatoes (79 cents versus $1.19 here), tunafish, crackers, chocolate chips (a 40-cent saving on a 12-ounce bag) and bottles of soda. Because of the time she would be on the road, she purchased nothing that required refrigeration.

For Goldwyn, co-op food buying is "more trouble than I want to get into." She boycotted beef a few years back, but isn't taking aim at any one product these days "I don't think we've really changed what we eat, but I do take a lot more care with what I buy.

"I was in Florida for a month and when you've been away for a long time, it becomes very apparent how high food prices are here. I expected things to be cheaper in Florida, but in New York?"

Sheehan who writes for the New Yorker magazine, is in the city often and will return by plane with legs of lamb or veal as well as nonperishable items.

Some foods, such as Hungarian cold cuts and baked goods, she buys there because the quality is superior or because they are not available here. Others may be cheaper, she theorizes, because they are in demand by the city's ethnic populations.

"But how do the supermarkets here excuse the price of coffee or tuna fish?" she asked. "They're the same cans. They always get off the hook on that."

Local consumer advocates have pointed to the lack of competition - or concentration - in the supermarket industry here as a cause of high food prices. Two chains, Safeway and Giant, are said to control more than 60 percent of the market in the city and its suburbs. In an annual survey conducted by food editors across the country, Washington has had the first or second most expensive market basket in the continental United States for the past several years.

The chains answer that labor costs are very high here, as is real estate; that sophisticated local shoppers allow no compromise in quality; that the East Coast is at the end of the distribution chain for vegetables, beef and numerous processed foods.

One alternative Sheehan and Goldwyn have adopted is multiple-store food shopping. They both cite Magruder's, which has only three stores and therefore is too small to be considered a "chain," as a place where they find bargains. Sheehan claimed she had priced 20 items in the Connecticut Avenue McGruder's and found them all cheaper than in the large Safeway nearby.

How real are the claims of cheaper prices, either in New York or in stores other than the major chains? How real is the feeling that food prices here are all going steadily upward?

The large chains would insist that their entire inventory, perhaps 10,000 items, be surveyed to give a fair picture of pricing policy. Falling short of that, the 35-item food editor's market basket was used for a survey undertaken early last week to compare prices at Magruder's and the neighboring Safeway on Connecticut Avenue. In addition, Safeway's prices of Aug. 15 and June 1 were compared, and the Juene market basket was used to compare New York and Washington prices.

Even in black and white, the results are neither totally black nor white. Using the lowest price for each item among three competitors in each city, New York's total was $37.72 to Washington's $38.99, or a difference of $1.27 (3.3 percent). The statistical message seems to be that comparison shopping makes sense, particularly for specials. But it would take a huge appetite for coffee, wieners and mayonnaise to make a grocery shopping trip to New York City pay for itself.

Safeway's market basket was under Magruder's by 23 cents ($40.35 to $40.58) for 34 items. (The 35th, Special K cereal, was omitted because the specified size was not available at Safeway.)

A breakdown showed 18 items higher at Safeway, 11 higher at Magruder's and only five with the same price. (It may be worth noting that in the June market basket, 18 of the 35 items were priced the same in Safeway and Giant.)

Among the better buys at Magruder's were: milk (10 cents less per half gallon), Grade A large eggs (16 cents), 10 ounces instant coffee (20 cents less), hamburger and sirloin steak (each 10 cents less per pound), carrots (24 cents less per pound), cabbage (14 cents less per pound), and iceburg lettuce (26 cents less per head). Cracker Barrel cheese, sugar, mayonnaise, evaporated milk, wieners, chicken, potatoes, bananas and oranges also were cheaper.

Safeway prices were distinctly lower for: 5 pounds of flour (20 cents less), Great Northern beans (14 cents less per pound), rice (20 cents less for two pounds), peanut butter (30 cents less for an 18-ounce jar), ice cream (10 cents less for a half gallon), frozen broccoli spears (18 cents less for 10 ounces), rump roast (10 cents less per pound) and butt end of ham (40 cents less per pound). Margarine, canned pineapple and frozen orange juice concentrate were also lower.

Shopping Safeway against itself 10 weeks later, tha market basket was up 90 cents, or about 2.2 percent. (That translates to an annual increase of more than 11 percent.) Prices had gone up on 11 items, but had declined on nine.

Among the gainers on the big board were eggs (up 10 cents per dozen), loin pork chops (up 10 cents per pound) and tomatoes (up 11 cents per pound in prime season). Special K, sugar, mayonnaise, evaporated milk, orange juice, lettuce, bananas and oranges were up as well.

Rump roast and sirloin, each down 20 cents per pound, led a general decline in beef prices, though ground decline in beef prices, though ground beef remained the same. Ham, bacon and margarine were each 10 cents per pound lower. Milk, cheese, frozen broccoli and chicken also cost less.

As for the Washington - New York comparisons, in June there were bargains to be found in both cities. Meat tended to be higher in New York, 30 cents a pound higher for ham and bacon, 20 cents a pound higher for rump roast. Potatoes were 10 cents more for five pounds; a head of lettuce was 9 cents more.

Here in Washington the consumer paid more for instant coffee (70 cents for 10 ounces), for mayonnaise (30 cents per quart although it was on "sale" here), tuna fish (24 cents per 6.5 ounce can), wieners (30 cents a pound), sugar (20 pound box) and flour (14 cents for 5 pounds).