TESTING SUPERMAN
I listed all episodes that come to mind and have some instance of a testing or a villains' manipulation of Superman.

BARBARIANS AT THE PLANET
Mrs. Cox tested Superman with green kryptonite at a false bank alarm.

RETURN OF THE PRANKSTER
Kyle Griffin tried to test his freezing machine on Superman, but it didn't work the first time. However, he broke into Lois' computer files and found information to revamp his device to make it work successfully the second time.

CONTACT
Not exactly testing, though, Superman was "forced to rescue" Lois while crimes were committed.(??)

ULTRAWOMAN
After hitting Superman with a red Kryptonite beam, the Newtrich sisters wanted to test the result of their experiment, hence caused a fire at the Daily Planet.

HOME IS WHERE THE HURT IS
Mindy Church and Bermuda staged Mindy's sucidal attemt to spray him with Kryptonian virus. To further confirm the results, Mindy varoomed in her Santa attire during "Superman brings Christmas tree to Orpahange" function.(??)

TEMPUS ANYONE?
The dangling of Lois from a ledge was to test if Clark would make a "come-out", a test orchestrated by Tempus

SECONDS
Lex Luthor staged various prank help calls / rigged buildings to hamper Superman's stopping him.(??)

LETHAL WEAPON
As LauraBF suggested, at least the first incident during the strip poker game, was a test by Jerry White to test the effects of Red Kryptonite on Superman.

STOP THE PRESSES
The Press brothers staged various high powered accidents to learn about Superman's recharging abilities.

Quote
Or am I thinking of a completely different episode than hers? I have this nebulous impression of a scene where someone isn't actually testing him but is trying to see what his reaction is to something. I just can't pin down who or what it was.
Could it be -- Stop the Presses?

If she had to move heaven and Earth, perhaps come back to haunt Perry and explain the story after they'd killed her, she would do it.

Waking a Miracle by Aria