You can't tell that from Activity Monitor. If you're truly worried that you may have some kind of malware download and run the free utility ClamXav.
Also, make sure you run software update from > Software Update... as this includes some security protection tools (including detecting and removing Flashback).
I don't really disagree with what has already been said. You can spend hours Googling all the processes listed to see if they are legit if you have the time and hope none of them are fake.
About the only process you can quickly identify as malware would be one who's name starts with a ".", indicating it"s a hidden file.