Rear end

Be careful checking the rear end like rumblefish described. Because sometimes, an open differential will get enough gear oil resistance that it will turn both wheels the same direction. The way I do it is, raise the rear end, put it in park or in a grea if it's a manual and try to turn the rear wheels. If they will not turn either way, you have a sure grip. Of course it's possible the cones or clutches could be worn slap out and it acts like an open diff and turns the other tire the opposite direction, but that's pretty rare.