"We had him throw today and we felt real good about what we saw," Coach Dave Campo said.
Campo said earlier in the week that if signed, Leaf would come in as the No. 3 quarterback behind Anthony Wright and Clint Stoerner. The former starter, Quincy Carter, is out with a hamstring injury.
Leaf had three disappointing seasons in San Diego with several injuries. Shoulder surgery forced him to miss the entire 1999 season. He started nine games last season passing for 1,883 yards, 11 touchdowns and 18 interceptions.
The Chargers, who had great hopes for Leaf when they drafted him with the No. 2 overall pick, right after Peyton Manning, waived him last March.