Need advice, can you help me get my sex tape back?

I made a sex video with a girl using photobooth on a mac. She then locked her computer and refused to give me a copy (psycho). After a long argument/negotiation she said she deleted it but I'm not sure I believe her. I'm confident I can get to her computer when it's unlocked. How can I either (1) find it and copy it, or (2) make sure it is deleted? Thanks everybody.