tom what I do is just DUST a coat of black or what ever you've got lying around (matt paints is best if you have to get some) over your filler work and primer, then just go over it with some wet and dry using a block where appropriate, what you'll find is the black will stay in the low spots and give you a good visual to what areas need more work.
and the other thing I would just say is always keep looking at the bigger picture I.E the whole door or what ever panel it is your working on and don't get caught up in just working on one focused area.
hope that helps a little? best of luck buddy
trac