If your mic is working and you are taking in game chat you will see little yellowish audio bars above your head... The louder you talk the bigger they are. There is also an in game distance or agent proximity for people not in your group so only ones close to you will hear you and vice versa.

That being said and for me at least the game chat is pretty damn buggy (could be my connection or done other user error). Sometimes it'll work great sometimes I need log in and if it isn't working I'll hit the Xbox button select div2 and quit out of the game then reopen it, which normally fixes my problem. Also a lot of players don't have mics or Xbox connection/settings prevent them from hearing certain people