if you mean like what part touches what (arm touches leg) then it is not possible.
otherwise if you just want to know when two things touch, you can do any of the following:
[ENGINE]
pfn_touch(ent1, ent2)
register_touch("classname", "classname2", "your_function")
[FM]
register_forward(FM_Touch, "your_function")
pretty sure that explains it (if you need more info, ask)