Yeah, you can check the effects of the gems in the Workshop without having to buy. You'll need some cash, but you'll get it back. Here's how:
(1) Save before going to the workshop.
(2) Buy one of whatever gems you're curious about.
(3) Select an item to upgrade. Put a gem in each slot you desire (max of 3 different gems, one of which has to be a Runestone). Press and hold the R1 shoulder button. Voila! It tells you what you'll get. If you don't want it, no harm done. Just re-load from you save.
If you want to know more, Interplay has a killer forum that goes into this and other matters in detail:
http://forums.interplay.com and click the topic "Baldur's Gate: Dark Alliance 2 Tips, Tricks, and Spoilers"). Have fun!