How to know when a guy is masturbating?

I think my male friends masturbates while we talk on Skype.
I can see him making those weird faces and breathing heavily, he doesn't answers me at the time.
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lolzz he really is weird the thing is he tells me I make him horny etc I asked him how horny he is he said up to 80% haha
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he makes faces like ummm like he is pushing something heavy like he forces his eyes shut etc
How to know when a guy is masturbating?
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