How to check engine codes?

I know I dont have a sentra, but both my mom and my girlfriend have sentras, both which seem to be running quite strangely lately. Moms car under warranty so thats goin right to the dealer, but girlfriends car is kinda messed up. Pickup has gone down and its burnin oil kinda quick.. i think it may be the O2 sensors. Can somebody please tell me how to get the checkin codes? I know with mmost american cars u can put a wire in the diagnostics under specific jumpers and turn the key to ON and it will reveal the engine error codes, but i have been readin on how to get it off this 96 sentra, and it makes no sense to me. Unscrewing the entire unit and turning it?? Somebody please make some sense of this for me. Thank you