tough, but not impossible.
I look for things like worn front suspension, wear in the receiver, signs of rear spring bushing wear. Also, a log book is nice to browse through. If owner notes down where service or fueling is done, and it's all over the map...
Another thing to look for is if the truck had gauges/tuners added. Not bad in it's self, but could indicate a kid hot-rodding, or someone trying to maximize tq/hp.
On the autos (I bought a 'o3 w/ 48re after asking q's here and on the forum I linked), if used for towing learn how/when to NOT use o.d. and when to lock the converter.
If you don't mind shifting get the nv, and others may not agree, I think the 'o3 early 'o4's would be the prize to look for.