Today we touched on a very touchy but extremely important topic. Halacha allows and even expects people to take background checks of there employees, business partners, spouses before getting married and the like before they get involved in a relationship like those. This is permissible because you are doing it for your own good will, in order not to get hurt, or to wind up in major arguments and fights which will lead to a chillul Hashem. This is not lashon hara because your intent is not to denigrate anyone and you are not placing a stumbling block in front of the blind, lifnei iver, by trying to dig up dirt about a person and asking questions which might get negative answers because this is all for the sake of your own well being. However you are forbidden to decisively believe what you hear, just act with caution and the person talking cannot exaggerate and can tell you only things which are needed for your well-being in this potential situation. For that reason also you cannot consult an enemy or even a competitor of the person you would partner with, hire, etc.because he will for sure lie or at least exaggerate since he hates him or is in competition with him or her. Even if he says he is telling the truth and is only warning you for your own good not because he hates him, you still cannot believe the guy because deep down inside he has negative feeling towards him so you must dismiss what he says if you accidentally hear something from him and certainly try to stay away.
The source of this halacha is based in a gemara in Shevuos daf 39b which discusses a person who entrusted money with another and the entrusted claimed he lost his money but the owner of the money claims he stole it, forcing the entrusted to make an oath. The gemara says they are both punished for what transpired. Rashi there says that the owner should have looked into the person before entrusting him with his own money. The Chofetz Chaim explains that they are both punished because if the entrusted swore falsely that’s a grave sin and if the entrusted was telling the truth the own forced him to make an oath for no reason, either way it resulted in a chillul Hashem which could have been avoided potentially if the owner would have just done the proper research before entrusting his money to this man.