How can I find out if my Girlfriend is cheating on me?

[Nathan]

She cheated on me, then said she's done with him. Now I'm suspicious, because she won't list me as her boyfriend on Facebook, and the guy she cheated on me with says he's still dating her, but she denies it!

[Jake]

Wow, dude this won't work! you gotta dump her! She probably is secretly dating him. Idk why she won't tell you the truth. You can get better!

[Kaitlynn]

she is still with him. im a female and i did pretty much the same thing to my ex. i found someone else but was still dating my ex bc i still loved him but wanted more then what i got out of the relationship so i decided to have a bf on the side.. and also ive playd the ggame many times and once a cheater ALWAYS a cheater.. so you gotta cut her off she is playing you! i mean shit she wont add you as her boyfriend on facebook thats a RED FLAG!. i mean there is your proof that she is cheating why else wouldnt she put you as her bf on fb.. seriously come on mann wake up lol