You can usually tell if it's lugging too hard or straining a lot. Then just drop to the next lower gear and proceed to the top. It should pull easier in the lower gear and not have to work as hard. That being said, don't over rev it then either. You should also be able to tell if it's over revving or winding too tight. If you've driven the truck a lot you should have a feel for it and be able to tell where it's "comfortable".
Hope that made sense.
