How can I tell I have a bad thermostat instead of a bad water pump?

Replace the T-Stat... IF that don't fix it ... Bound to be something else!!!

If you are loosing a lot of coolant have your pump inspected and all coolant lines. Can you see where fluid is coming out?

If you are overheating and the fluid is boiling out and there are no external leaks, which you can test by using a pressure tester. There is a possibility as well of a bad head gasket or crack in the block or head. If a thermostat does not fix it I would have it evaluated by a shop. Good Luck Clint