You'll probably wan't to use a structure like BSP Trees or Octrees to limit the number of polygons you're testing against, then you use simple Polygon->Polygon intersection tests or Ray->Polygon. There is a lot of information on intersection tests so I won't go into it here.

--TheGoop

what's polygoin->polygon testing? check each polygon from oone object to every polygon of another polygon?

how do you do collision for 3d levels?

I don't think you realize the scope and size of the question you are asking. Maybe there should be a resource available for this because we seem to see this question come up all the time. Is there? Anyway collision detection in a 3d world is a big topic because there are as many ways to do it as there are ways to program.

I would suggest searching the web for articles. Go to www.ga-source.com and this site looking for articles explaining it. Effectively it is the art of testing if a polygon in one 3d object is going to intersect another polygon in a seperate 3d object before you redraw the screen when the polygons are moving along vectors AB and CD over time t at speed v. The other thing you could look for is an advanced geometry book as all of the theory or examples you will find are derived from math like everything else in programming.

Hope this helps you out.
Kressilac