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

[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