If a person is unsure as to whether or not something is Haram but he doesn't check and does it anyway is he sinning ? What if the same scenario was applied to something that takes a person out of the fold of Islam. Does he become a kaafir then ?

Asking is obligatory before you act, it is not permitted to stay silent and go ahead upon ignorance, and whoever leaves off the doubtful has protected his religion. As for the second, generally, an act may be kufr while the man himself is not ruled a kafir by name, that is for the courts and the muftis, it is not for you and me.