I took a look at her belly while we were playing this afternoon
Did you notice anything abnormal or any other body parts?
I looked at her thighs, I think they're looking thinner than usual.
Well, you just told me that this was a friend which means that you inspecting her body while you're both playing a game may creep her out as it is can be considered quite sexual in nature. Would you like it if a random male friend started staring at your thighs?
She's my best friend and I just want to know if she's okay with me.
Well, most people would say that you should attempt to speak with her if you want to know if she's okay with you and that staring isn't the way to go about it.