Sefer Chofetz Chaim Hilchos Rechilus chapter 9 halacha 2 conclusion of footnote 9

It’s obvious that if you don’t personally know, for example the maid, that she is a thief, or any other example of the like, rather people just told you, even though we said you can tell the would be hirer so that he can protect himself, however you can’t just say plainly “so and so is a thief” looking as if you know yourself this fact. You can’t even say “I have heard that son and so is a thief” because many people, not being experts in the laws of accepting rechilus will take it as fact. Rather you should be as non-confident as possible and say, “I heard about so and so this and this, and maybe the matter is true, therefore t would be appropriate for you to be cautious and protect yourself from her.” This non-confidence is the way to act in all circumstances of the like to protect others besides a person looking to hire a teacher or looking for a shidduch and the he or she is rumored to have fundamental flaws in his or her Torah perspectives, philosophies or beliefs (not observance), which could be serious red flags, then you can just say with more confidence “I heard…”

The reason why rebuking the perpetrator beforehand is not on the list of conditions as it was by lashon hara is because it’s not applicable here. One is looking to hire someone else. It doesn’t make sense to first rebuke the would-be candidate before definitely deciding to go to the hirer to warn him the candidate s a thief because even if he promises he will change who says that is true, the hirer still has a right to protect himself and know what he is getting into. Whereas by the laws of lashon hara, one should first go over to the thief and rebuke him because that was in order to get the thief to give back what he stole without needing to tell the victim what happened.