Start by checking for flow. Feel the lower hose to see if it is hot. If not, you do not have flow. Clogged radiator can do that but this is not as common as a stuck thermostat. I have seen one water pump im my life that lost it's impeller, but that is very rare.
You can test your thermostat (new or old) in a pot of hot water on the stove. If it opens at 180, or what ever the temp rating on the stat (before boiling at 212) you are good to go.
-Mark.