Hi, about private surfing, and checking

Hi, how do I scan what someone has been , on the net...

and also how do I stop them, do they use cookies or ports? pings or ripe to find out what I surf, I'd like to know what my computer surfs when I leave it for a moment

 

Why are you worried?

Do you use a firewall, anti-virus/trojan/spyware tools?
If so, and yuo manage them properly, no harm can be done. Your computer is yours and yours alone, unless you allow others to own it.

Just pinging or port scanning does not compromise a system.