Its easy, you test her.
Watch how she treats other people.
Set up situations where you can guage her reactions. Observe her interactions with her family.
Heres an idea, let your phone run out of charge some random day when your out with your girl and tell her you need to text your family member back, or friend or whatever and to use her phone. If she hesitates at all...lol well well well what do we have here?! Remember do this out of the blue when everything seems most "perfect". Do not do this unless you are alpha and can handle the consequences. Me personally, I hold all emotion of a woman until she passes all of my trust tests...then she can start getting some feels. Other then that...I treat her like an associate of any type when it comes to anything involving important trust.
This is only to be used on a honey who claims she loves you/is 100% down for you only, wants pure exclusivity, and has a proven track record of being a decent person and showing loyalty. I have to add that part in there because I know alot of you simps will be trying to go through the phones of whatever girl your taking out to date number 2 at applebees or whatever or wanting to use the idea to see what chads your crush is sukking off. this isnt to be taken lightly as I dont believe in invasion of privacy but then again when it comes to personal security its better to have knowledge of what your dealing with.
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If you see some chit in her phone that you dont like. You need to remain cool calm and collected bro. There is zero need to get emotional about this. Its going to go one of two ways, she nonchalantly hands you her phone without a care in the world (because shes truly innocent and means what she has said and or she is confident she has scrubbed her phone clean of all dirt!). Either way unless you have other suspicians, it should be all clear for the most part. be vigilant of course and guard your heart but you can show some care toward her if you are convinced shes genuine.
Now the second way it can go is she hesitates and refuses to give up her phone or makes some crazy excuse like her text isnt working or WHATEVER...shes not giving the phone. Again be calm cool and collected. Just look her right her eyes and into her soul and open yourself up. let her know whatever it is, you will not get angry with her and that we can just be 100% honest and that whatever she says you can handle it as you are a grown man and shes a grown woman so as adults we can talk about it.
If she has balls she will tell you the full truth, ie shes bangin chad an tyrone...or shes just got some flirty texts with random guy from work/school/the club etc. She might even down play it at that point to him being a creepy stalker/orbiter whatever. If shes a cruel kunt she will flip it on you and say your trying to trick her, you dont trust her, why is YOUR phone dead and what are YOU hiding etc etc.
Either way you just have to ask her to see whatever she is talking about because if we are being honest its the only way we can possibly move forward, heck..the texts might not even be anything...right? maybe because her exbf was jealous an crazy that you might react that way because her classmate asked her about homework. You never know bros so thats why do not react. be logical and clear. transparency is the key.
When you find out the bad news. you go cold on her. show her it was fked up and she did you dirty. however let her down easy. drop her off or split ways and just say "hey it was nice to have met YOU...if that was ever even the real you...regardless...I wish you well but this isnt going to workout with me...to me loyalty is everything and you really fukked that up...maybe i can give you another chance maybe not..your actions are going to prove it either way. Bye"
then you walk off into the sunset and keep her as a plate if you want to bang her and make her work for your affections (which is wat real players do...instead of getting emotionally involved)...you just have to look at her as someone who IS going to fk up on you eventually in different ways...you just need to kno how to play the game right to come out on top