Identifying suspicious processes

I am looking for help in removing trojans or malware that may have crept into the system of my MacBook Air.

As I've posted elsewhere, I recently downloaded (accidentally) the Yahoo redirect that hijacked my Google searches in Safari. I removed the associated extension but I think there may still be unwanted files/processes on my machine. One reason I suspect this is that I keep occasionally getting an "Apple wants to make changes" window requesting my password. It's happening when I'm not doing anything that should prompt that.

I have read all the good advice about not using software to "clean" my Mac, but I would appreciate advice about how to identify and manually remove unwanted objects/scripts that I can spot via Activity Monitor, for example. Or anyplace else I should be looking.

Many thanks!