Sharp-eyed home-buyers and owners can pick up superb mortgage-money bargains in the current marketplace -- loans that lenders probably shouldn't be making -- provided they know how to spot the telltale signs. That's the advice from an astute money market insider here who analyzes mortgage rates and terms offered by lenders across the country.

David Andrukonis is an economist for the Federal Home Loan Mortgage Corp. and an expert on what mortgage money costs and why. He conducts the corporation's surveys of what 125 large and small lenders are asking consumers for new home loans.

The patterns and opportunities he sees emerging in the capital market could be useful if you're planning to refinance your home or get a new one in the near future.

To spot true bargains this month and next, you have to understand how borrowers and lenders have reacted to the nearly 2-percentage-point downward flutter in interest rates since August, according to Andrukonis. Many borrowers have turned up their noses at the adjustable-rate loans that dominated residential real estate for much of 1983 and 1984. They've switched to fixed-rate, peace-of-mind mortgages instead. The latest government studies, in fact, put adjustables at barely 50 percent of the market, down to their lowest share since mid-1983.

A second key fact for savvy borrowers to understand is the true cost of mortgage capital, as defined in the national secondary marketplace. The secondary market, the source of the lion's share of capital for American home buyers, consists of deep-pocket investors such as pension funds, life insurance companies and others who purchase local home mortgages wholesale, packaged together like sardines in the form of securities. Freddie Mac and its uptown relative, the Federal National Mortgage Association, are active buyers, brokers, packagers and investors in this vast capital marketplace.

The current secondary-market cost of fixed-rate mortgage money -- what the big pension funds require as a yield on mortgage securities -- is a hair under 13 percent. With Freddie Mac's and local lenders' fees tacked on to the deal, the end cost is 13.4 percent.

A third fact you need to know: The average "spread" nationwide between yields on one-year adjustable-rate mortgages and traditional 30-year, fixed-rate loans is 2.2 percentage points. A lender offering typical 13 percent, 30-year mortgages, in short, is likely to be offering one-year adjustables at slightly less than 11 percent.

Take these three pieces of data, Andrukonis advises, and ask yourself: How long am I going to live in the house I'm buying or refinancing? In some parts of the country, the average is hardly five or six Another bargain is fixed-rate mortgages offered by lenders who believe rates are headed further downward years, opening the possibility of taking advantage of discount, short-term rates rather than 30-year al- ternatives.

Whatever your answer, you're now equipped to look intelligently for bargains. So where are they now?

Andrukonis points to several promising areas. First, his surveys indicate substantial numbers of three- and five-year adjustable loans being offered that come with rate "caps" or ceilings that effectively turn them into fixed-rate bargain deals for borrowers.

He says that a commonplace example of such a mortgage is an 11 percent, three-year adjustable with a 3-percentage-point cap. Even under the worst conceivable economic circumstances, this equates to an 11 percent, fixed-rate mortgage for three years, and no higher than a 14 percent, fixed-rate mortgage for the second three years. If rates dropped in 1987-88, it easily could turn into a 10 percent or lower fixed-rate loan -- not bad for a buyer staying in the home for a typical residency period.

Another bargain found around the country is fixed-rate mortgages offered by lenders who believe rates are headed further downward this year, a belief Andrukonis suggests is fallacious. With the true secondary-market cost of capital more than 13 percent, "anyone originating fixed-rate mortgages at 12 1/4 to 12 1/2 percent today is betting the wrong way," he says.

Those lenders may be pulling in lots of new business, in Andrukonis' view, but they're also giving away the store. Check your local marketplace with Andrukonis' suggestions in hand. Spot a lender who's betting the wrong way and you may walk away with a little piece of the store.