Know of any websites that will identify various lugnut spacing measurements, or that can verify if certain wheels will fit other vehicles? Short term, I need to know if a set of 2005 F-150 FX4 18" wheels will bolt on to my 1996 F-150...

 

here's an article about measuring lug patterns, etc. You just need to know what pattern each truck came with, or measure both if possible.
Don't know if that's exactly what you're looking for, but hope it helps.

Wheel Article

 

The new F-150's have a 6x135mm bolt pattern. The only vehicles with that pattern are 04-07 F-150's and 03-07 Expeditions.