The Washington Diplomats will get a much better idea of how good they are today when they play host to the two-time defending NASL champion Cosmos in a National Conference Eastern Division battle at RFK Stadium at 2:30.

"This will do it," said Diplomat defender Robert larusci, who played on both Cosmo title teams before coming to Washington this year. "This game will give us a good indication of where we stand.We haven't been extended yet and, against the Cosmos, we'll be able to make comparisons.

"I've known the pressure of playing against them and the pressure of playing with them. They are a beatable team," said Iarusci, who also was a member of the Toronto 1976 NASL championship club. "They've got seasoned pros and are not weak anywhere. But just playing against them makes your skill level come up. Teams always got up for us when we came to town."

A crowd of 25-27,000 is expected. More than 17,000 advance tickets have been sold for the division's first-place battle. The Dips are 2-0, have 17 points and will have Paul Cannell in action for the first time this season. The Cosmos are right behind with a 1-0 mark and eight points.

"It's not a must win or the turnabout game of the season but both teams would like to win this one," said Iarusci. "They'll want to beat us and will go all out."

Iarusci and his back-line mates-Don Droege, Tommy O'Hara, Mike Dillon and goalkeeper Bill Irwin-will have their hands full shutting down the offensive-minded Cosmos.

Giorgio Chinaglia, who established a league Record for goals (34) and points (79) last year, is especially, dangerous anywhere near the box. Dillon, who will have the responsibility of stopping Chinaglia, was burned for three goals by the Italian striker in the Cosmos' 6-1 victory over Washington in Giants Stadium last year.

"I marked (guarded) him last year and he got three goals in the second half against me," said Dillon, who also played with the Cosmos for three years. "He's tough if he gets his service from the midfield. I have to make sure he doesn't get the ball. He gets frustrated when he's not getting the ball and starts yelling at his teammates. When he's not getting the ball as much as he wants, he'll go out in the middle and get it. When he does that, I'll go right with him."

Chinaglia is only one weapon in the Cosmos' stocked arsenal. Wingers Dennis Tueart and Seninco are quick and explosive. Midfielders Franz Beckenbauer, Vladislav Bogicevic and Marinho come up very quickly on offense, which, in effect, gives the Cosmos six forwards.

"If they have one player we must worry about, it's Beckenbauer," said Washington Coach Gordon Bradley.

The Cosmos struggled last week in beating San Diego, 21, in overtime. They had problems on defense and only good luck prevented the Cosmos from losing. Socker players missed at least four point-blank attempts and had one goal nullified because of a questionable official's call.

Washington, on the other hand, has looked very sharp in its first two games. The offense did as it pleased in both Diplomat victories while the defense has faced very little pressure. Goalie Irwin has had to make a total of only eight saves.

Forward Alan Green had two goals in the Dips' 5-1 thrashing of Atlanta last week. Washington reeled off 31 shots compared to the Chiefs' 11. In two games, the Dips have outshot their opponents, 52-19.

Joining Green up front are Bobby Stokes and Sonny Askew, who also scored his first goal as a pro last week.

Midfielders Joe Horvath, tied for the NASL lead in scoring with six points, Sakib Vietskic and converted sweeper Jimmy Steele looked much more settled last week compared much the 2-1 overtime win over Fort Lauderdale two weeks ago.

"That's where we have to play well. We have to control the middle of the field," said Bradley, who coached the Cosmos for seven years. "We have to command the game in the middle. We have to mark the three men up front very tight, also. We have to keep pressure on them and not play their game-make them play us."

All children under 14 will receive a soccer ball. . . The Cosmos hold a 6-3 advantage in the overall series. Washington has been outscored, 37-14, in the nine games. . The Cosmos took 27 shots against San Diego last week; the Sockers also had 27 attempts. . . The Cosmos defeated the Dips twice last year, 6-1 and 2-1. It was reported in Saturday's editions that the Dips won the second game, 2-1. It wwas the other way around.