Washington — Who won? Who lost? That will surely be the No. 1 question after Sunday night's first debate between Ronald Reagan and Walter Mondale. But the answer isn't always obvious - even to those who take part.
The winners-losers question is one that many political experts answer with trepidation. Even they can be fooled.
History, however, does offer us some useful and sometimes surprising pointers with which to measure the impact of presidential debates. Remember, for example, the debate on Oct. 6, 1976? That one really surprised the political insiders.
The subject was foreign policy. The opponents were President Gerald Ford and his challenger from Georgia, Jimmy Carter. Mr. Ford strode into the debate feeling confident. Foreign policy was his bailiwick. Mr. Carter's aides were anxious.
As the debate took place, Ford felt he scored again and again on important issues. Afterward, his aides congratulated him. The President went to bed feeling great. He was sure he had won.
He was wrong - even though early opinion polls showed that Ford had beaten Carter in the debate by 11 points. Yet only 24 hours later, private Republican polls found that Ford had lost by a margin of 45 percent.
The reason: a single slip of the tongue by Ford. The President had misspoken and said that Poland was not under the domination of the Soviet Union. The public had barely noticed the comment. But the press did - and reporters jumped on it.
The public, initially impressed with Ford's performance, later read press accounts and realized that the President had goofed. Even voters who had watched the debate changed their opinions. The public's flip-flop stalled the Ford campaign, befuddled the President, and was eventually seen to be a major factor in Ford's narrow loss to Carter.
The Ford-Carter experience gives us one gauge of judging winners and losers: How does the press play it? Although the public doesn't always trust the press, people still look to columnists and reporters for further information in forming their own opinions.
For viewers, there is no easy 1-2-3 method for picking winners. But political experts, including Dr. Austin Ranney, editor of ''The Past and Future of Presidential Debates,'' do offer a few helpful tips:
* How does the press report the debate? It takes as much as 48 hours for the full effect of newspaper, TV, and radio reporting to be felt. Polls taken immediately after the debate aren't always reliable. If press commentary focuses next Monday and Tuesday on ''Reagan's gaffes'' or ''Mondale's blunders,'' that will be a strong indicator.
* Who sets the agenda? Many voters will recall the famous Kennedy-Nixon debate in 1960 when Richard Nixon followed the script, and stuck rigidly to the questions he was asked. John F. Kennedy breezed past the questions and delivered his own campaign messages. Academically speaking, Nixon ''won'' the debate; but Kennedy made a better impression on voters. Each candidate on Sunday will want to ''take over'' the debate.
* Who seems more relaxed? In 1960, Kennedy appeared calm and cool, even though he was up against an incumbent vice-president. Nixon was sweating and looked wan. Most voters who listened to the debate on the radio thought Nixon had won; most who watched on TV gave the advantage to Kennedy. Both Reagan and Mondale are experienced debaters, and Mondale had lots of warm-ups in this year's primaries.
* Do the candidates give complete, detailed answers? Richard B. Cheney, who was chief of staff in the Ford White House, says that scientific studies during the '76 debates found that voters were far more convinced by longer answers. A short, incomplete answer was not enough to overcome the bias of someone against a candidate; but a longer response got the voter to think and reconsider.
* Whose answers are more vivid? Reagan is known for his down-to-earth examples. Mondale is more formal, but occasionally cuts loose with a vivid line. Remember Mondale's ''Where's the beef?'' gibe at Gary Hart during the Atlanta debate? Those three words threw Senator Hart off his stride for days.
* Do the candidates use humor well? An ill-considered joke, of course, could be deadly. But humor at the right moment gives the impression that a candidate is in charge of a situation. Kennedy (1960) and Reagan (1980) were prime examples.
* Do the candidates avoid mistakes? Ford's misstatement on Poland stands as the classic example. It reinforced his media image as a blunderer and head-bumper. Some commentators are suggesting already that unexpected gaffes could play a major role in the Reagan-Mondale debate.
* Do the candidates overreact? Being too strident, becoming angry, launching a personal attack, shouting - all of these are obvious no-nos'. The image should be polite, firm, and presidential.
One final point. Any debate is considered advantageous for a challenger, like Mondale. It puts him on level ground with the President. It makes the President ''just another candidate.'' So if the outcome is even, Mondale should gain at least a little, the experts say.