''We have observers out now, and we're always watching tapes looking for competent new officials,'' Thorn said. ----No Chance for 70 Victories With the Los Angeles Lakers dominating play, speculation was growing that they could become the first team in league history to win 70 games during the regular season, beating the 69 victories they posted during the 1971-72 season when they won a record 33 in a row. But 10 minutes into the game against the Chicago Bulls on March 10, Magic Johnson left with a strained groin muscle. Johnson has not played since, missing the last five games. The Lakers went on to lose the game to the Bulls, and three of the next five without his playmaking and scoring. The only victories were over the Nets and the Golden State Warriors, two of the league's worst teams.

The Lakers are 51-14 with 17 games remaining, so they now have no chance of attaining the 70-victory mark.

Johnson will be examined before tonight's game against the Houston Rockets. Michael Cooper, who has missed 12 of his last 14 games with an ankle injury, will have his injury checked.

After Sunday's 130-127 victory over the Warriors, Kareem Abdul-Jabbar acknowledged that the injuries have put added pressure on the Lakers. But without pressure, he said, ''you don't have diamonds.'' ----Woolridge in 'Good Spirits'

Willis Reed, the Net coach, spent an hour and half with Orlando Woolridge at the drug abuse center at Community Hospital in Van Nuys, Calif., on their recent West Coast trip. ''He was in good spirits,'' Reed said. ''He has been lifting weights three days a week and running every day.'' No date has been set for the high-scoring forward's release from the drug program he entered on Feb. 24. GOOD TO BE HOME

Since the Celtics defeated the Pistons, 118-111, on Nov. 15, 1986, at the Pontiac Silverdome, the home team has won the next 15 games between the clubs, eight in Pontiac, Mich., seven in Boston, including all seven games in last year's Eastern Conference championship series. That could have a major bearing come playoff time. The team with the best record in the conference gets the home-court advantage. Only one game separates the Celtics (46-20), the Atlantic Division pacesetter, from the Pistons (44-20), the Central Division leader. They will meet two more times, April 1 and April 19, both at the Boston Garden. . . . Kevin McHale (.611) and Robert Parish (.584) of the Celtics have ranked one-two in the league for much of the season in field- goal percentage. If that holds, it will be the first time players from the same team have held the top spots in that category.