GF Cheating?

While fingering my girlfriend she seemed really wet and silky inside, when I pulled out, my fingers were covered in this. I wiped it on my sock then continued. I have posted stained panties before wanting to know if people thought it was cum. Well this was deep inside her.. is it cum? I'm not allowed to bareback her and she has accidently said she has seen her girl friend naked (who lives with three single guys, and fucks when things get wild) "while changing before going out" but i know 100% she changes before she leaves her house. This would be her perfect opportunity to cuck me. Have included some of the old panty shots. Please tell me if what's on the sock is cum and what you think of the stains. Turns me on massively to think of her getting cock and cum up her but also feel torn. So, is it cum?
Young 9inch cock looking for creamy pussy. Love to fuck wives deep, love cum. Dirty gf for foursomes if girl is right.

she's getting some behind your back. that's cum buddy.

The only way you'll really know Dude, taste it. Smear it on your tongue and swirl it around in your mouth. If it tastes great and you get a hardon, it's cum. If not, spit it out, it's probably a diseased vaginal discharge. Good luck!!

Hi sally hear, that is cum and as a woman I should know. I fuck about with loads of guys and my panties look just like those.