I bought a used set and it has 5/6 bags. How can I identify what parts are in bag 1 so I can order them? Set is 7964
Lego's own building instructions will show you which bag you open when, so if you are willing to look at each step before bag 2 is used, they must have come from bag 1 (unfortunately, this includes minifigures).
I'm not aware of any LEGO related online catalogs keeping track of individual bag contents. So there is no convenient way of knowing what's missing.
One way is to do as mentioned by RSchulz, but I find it tedious to identify each piece and make a note of them.
If you are willing to open the rest of the bags you could check against existing set inventories on Bricklink or Rebrickable. Such way you don't need to identify the name (the most tedious, probably), part number and color. Only thing is needed is to note the quantity you already have.