I've had a number of comics (approximately 100) passed onto me, none of which I know anything about. I've no idea whether they're of any monetary value or not and I don't want to hold onto them.

How do I find out if they're saleable or not, and who do I sell them to if they are, without being ripped-off?

Here's a sample of some titles:

Predator Volume 1
Predator Cold War
Predator Big Game

Superman At Earth's End (Tom Veitch and Frank Gomez)
The Death of Superman
World Without A Superman

The Power of Shazam
The Power of Shazam (The Price of Liberty)

Contagion The Batman Chronicles
Contagion 1 Batman (The Shadow of Bat)
Contagion 2 Batman
Contagion 3 Robin
Contagion 11 Robin

Spider-Man Revenge of the Sinister 6
Spider-Man The Cosmic Adventures

And too many more to mention !

I think most of them are from the late 1980s to the mid-1990s.

Does anyone have any advice regarding value?

Check similar titles on Ebay... Might be your best bet.
Most of those sound like trades/graphic novels... I'm not sure places like http://www.silveracre.com/ buy those off folk but it 'might' be worth giving them a call...