Sign onto MSN/Yahoo and invite every online contact to view your webcam, then cover your monitor so that you can't see what is on screen and leave the room so that all your contacts will see on cam is an empty room.
Return to the room 5 minutes later and, without uncovering your monitor, strip naked and masturbate to orgasm. Now you may uncover your screen and check if anybody saw you.
If they got bored watching an empty room for 5 minutes and stopped watching, you are lucky
If you want to make this more risky, you could only leave the room for 2 minutes and when you have orgasmed, sign off without checking who saw