A trick that I've seen in scale modeling magazines... Cut a piece of plexiglas that's the size of your longest car, and attach a pair of trucks. Roll it around all of your curves and check the overhang on the inside and outside - mark the locations and look for interference. The use of a clear base can also help with noticing track imperfections. Sounds like a good idea, but I've been too lazy to do it.
The key with all of this is finding the locomotive or car that really has the largest overhang - I've learned (the hard way) that it's not always the car or loco you expect it to be. The item with the greatest inside overhang might not be the same as the one with the largest outside overhang. Long steamers can have a lot of overhang at the back of the cab.
Another lesson learned the hard way is that the overhang is slightly greater when running at speed, so the moral of the story is to not cut it too close.
