How about a curse on the locale they are adventuring in, whereby everything is turned on its head.
The traditional bad guys are the goodies, the usual goodies the baddies. Spells do their opposite. And - just for fun, guys, to reflect the topsy-turviness of the world - all To Hit rolls are Roll Under. All Saves are Roll Under. Any pluses become minuses. So your 1st level Prof Bonus is -2.
AC is reversed from standard bare skin AC10 so AC 15 becomes AC 5, AC 12 becomes AC8, etc.
To hit you need to roll UNDER the target score.
Damage is mirrored. So a roll of 1 on a d8 is actually 8 damage, a 2, 7; a 3, 6; a 4, 5, and so on.
If the cleric tries cure wounds he will actually inflict damage; if he drinks a healing potion he will lose HP - he should be necking poison.
If an area effect curse, everyone is affected equally, but while everyone else's random rolls are just as likely to work, you'll discover if....
If he's cheating, expect the same crits and max damage rolls etc. he will fudge, just fudge backwards.
If his dice are loaded, he'll struggle. All those 20s are now fumbles...oops.
It will soon become obvious what the real issue is.
Of course, he MAY just be a Halfling in real life.