In the old days, changing the date, deleting the preferences, or installing the trial software in a different path did the trick, but, nowadays, it seems rather unlikely that there's such an easy way.
Trial software may call home to check the database for a combination of your MAC address and your email, your IP, both, create an invisible file with a who-knows-which-name or a million different things. I mean, if there was an easy way to do so, you wont need the full version, right?
Someone might find a way, of course. But it will probably be for a specific program, not a generic way for all trial software.
EDIT: Just noticed that I'm not answering your question. Indeed, installed and used applications will leave other files, such as Preferences, stored either in your user's preferences folder (~/Library/Preferences) or in the System-Wide preferences folder (/Library/Preferences/), or Application Support Files (which will be, again, in your user's library folder or in the root one). There could be a lot more, of course.
If you want to get rid of them, making a search, with Spotlight or Finder, should show them all.
