HOW GOOD are the multi-billion-dollar weapons the Pentagon plans to buy? That subject was stirred up again the other day by public release of a year-old Army report on field testing of the controversial M1 tank.

Whether or not the tests show that the M1 is a wise investment for the $20 billion involved, it's the general question that is more interesting: is defense decision-making sufficiently open to the possibility that, no matter how much money has already been spent, experience may show that some weapons ought to be junked -- or greatly modified -- before more money is wasted?

For some years now, the General Accounting Office, special defense advisory panels and academic critics have asserted that weapons testing within the Pentagon is not rigorous enough. Test findings are ignored or explained away in the powerful drive within the bureaucracy and the defense industry to move weapons systems into full production as quickly as possible.

Realistic field tests are not normally conducted until after the decision to start weapon production has been made. Testing organizations within each of the services and at the departmental level have independent access to the people who will make the decisions, but by the time their tests are completed, much time, money and prestige have already been invested in the project. At that point, test advisers can't do much but insist on certain modifications, even if their findings call the whole weapons project into doubt. Reagan administration moves to give more control over weapons development to the individual services -- where the commitment to particular weapons is traditionally strongest -- may further reduce the testers' influence.

Pentagon managers argue that the system has a strong built-in check: the weapons it produces must ultimately be used by its own forces. But the actual field deployment of a weapons system in combat is far removed in time and responsibility from the decision-makers who guide the budget process. In any case, making sure that, for example, a tank won't burn up its occupants is only one aspect of determining that the best possible weaponry is being bought for the available money.

One frequent proposal, soon to be introduced as legislation by Sen. David Pryor, is to set up a test office totally independent of the rest of the Pentagon bureaucracy. That might help raise the visibility of testing, but it's not likely in itself to produce better decisions. What's really needed is a commitment by congressional committees and top Pentagon officials to ask tough and thorough questions at the right time and, further, to be prepared to accept inconvenient or disagreeable answers. That, of course, also means not worrying about how much money has already been misspent or whose reputation is on the line or whether the Pentagon won't get as much money as it hoped for next year.