Ensuring there is no snooping

Hi, is there a way to check and see if someone (like a creepy guy a friend lives with) has been snooping on a Mac? I know Windows well enough, but I'm new to Macs. I have a friend who is living with someone that we're pretty sure is accessing web based email and browsing history locally on her mac (well I assume locally). Because of the length of time and precautions we have taken, either my friend and I are just going nuts, or this guy has been able to obtain passwords and browsing histories...maybe more. I know there is software that can do this but the few that do exist seem kind of shady. I also know there are OSX admin settings that can allow for monitoring activity, but I don't believe they are sufficient to obtain passwords to access sites like Gmail. We delete cache and history after use. Local IMAP and POP3 clients are not used.

So, what I'd like to know, please, is what steps would a Mac expert go through to see what/how/if something fishy is going on? On a PC I'd download spyware software (which is rare in the mac world) like adware or spybot. That would probably do it. Then I'd just check system monitor, registry, and then change user pwords and login. I appreciate the help. Thanks.