WE USED to be able to spot a frozen vegetable at a glance, until the industry foiled those cursory glances with "random-cut vegetables." And we thought we could rely on a skin-on french fry to be a fresh-cut french fry, but that sly frozen food industry is at it again, this time making frozen french fries with the skin on . . . And that brings to mind the new, everybody-has-it appetizer, potato skins. We saw them recently on a restaurant menu which had no other dish that could be using the insides of the potato. So we asked what they did with the leftover insides. The waitress confided to us that they bought just the skins. Once word gets around, somebody is bound to try to develop a market for banana peels.

The White House mess, which is the upper-echelon staff dining room, has gone from dry to -- well -- damp. First there were margaritas and beer for Thursdays' Mexican meals. That was during the Nixon and Ford administrations. Then the Carter people turned off the spigot altogether. The Reagan people now allow beer and wine, and "cocktails are available." (The best we can make of that distinction is that one can have a cocktail if one must, but it is not encouraged.) White House press spokesmen report, however, that there has been no stampede; said one, "They're just not heavy drinkers in this White House."

We are worried. For the Postal Service. Word has reached us that supermarkets are switching from brown paper bags to two-handled plastic bags. And we wonder whatever people will use to wrap their Christmas packages for mailing next December. We also suspect attention will have to be paid anew to trash can design and to chldren's Halloween costumes, should those familiar and multi-purpose brown bags disappear.

An opportunity seems to have been missed in the promotion of the newest timesaver, carrot cake mix. Why in the world, we wondered aloud one day, would anybody need a carrot cake mix? It is just the thing, came the reply, to have on hand after a hectic day of jogging and meditating.

Red wine spilled by a waitress on our tablecloth set off the storyteller: One evening in Paris, said he (immediately capturing everyone's attention, for this was just Virginia), a waiter spilled red wine on his daughter's dress. Instantly, however, the waiter knew how to rectify the problem. White wine, he explained, removes the stains of red wine. By that time he was dabbing the endangered dress with white wine. It worked for both the dress and the waiter, who charged them for both wines.

In the strange bedfellows department, PepsiCo has signed an agreement with Japan's Federation of Hospital Associations to develop beverage products for their hospitals' dietary improvement programs. It brings new meaning to the term Diet Pepsi, and makes one wonder about the future possibilities for Fritos, which are also under the PepsiCo umbrella.

While Pepsi-Cola remains No. 2 to Coca-Cola's premier position in soft drink sales, Dr. Pepper has made a great leap forward this year, bypassing 7-Up to become the third most popular soft drink, which means 323 million cases sold in 1980 (to Coke's 1,425 billion). We wouldn't presume to know what all this means except that whatever soft drinks Americans are drinking, they are drinking enough to make your head swim.

Of all the edible temptations that cross our desks, a particular one recently caught our eye, if not our fancy. As we opened a three-page letter, it sprinkled dark green dust over a deskful of the usual slick and shiny press kits from the likes of Campbell's and fisheries' foundations. The green dust, the letter explained, is spirulina plankton, which its distributor hopes will turn children away from "sugar and lifeless food." This $30-a-pound sea dust is called by its promoter "the most potent form of natural nutrition on the whole earth." What he hasn't discovered is a better method of sending a sample through the mail than taping a wad of it onto a letter. We can't tell you how it tastes or what it can do for your minimal daily requirements. But at least we can tell you that it washes out of your clothes, and for that we are grateful.

It was on these very pages that we allowed it to be said that rhubarb is not eaten raw. Thus on these pages we are compelled to report why reader Janet Christoff misses the rhubarb patch of her Iowa childhood. For raw rhubarb, wouldn't you know? In her neighborhood they used to pick rhubarb and dip the end in a bowl of sugar, eating it thus in its natural state. We look forward to hearing from more Iowans during corn season.