I've gotten a headshot with a frag grenade, which surprised me. It seems their idea of a "headshot" is when all of the kill damage is directed at their head, thought it could be more complex than that. The only way to know for sure is to experiment.
No, there is no possible way to get a head shot with a semtex. You can get a head shot with any gun and grenade launcher but not with grenades.
i've gotten a headshot with a noob toob and i'm pretty sure the game gave me a headshot on a different occasion for some kind of explosion i caused but i'm not completely sure
i guess its possible if he stuck it so the guys head
If not sure, I do know that you can get a headshots with noob tubes but I don't know about grenades, I guess it's possible because it would stick to there head, id say that it's possible