Check them with a Pyrex glass container and heated water ... it's helpful if you have a thermometer to check the water temperature, as knowing the exact water temp will help you determine if it's working at the desired temperatures, but plain boiling water is OK since that is hotter than the opening temperature, so it should open completely if that is all you have.
You can add cooler water and watch it close for a basic function check.
Note that thermostats aren't like a digital switch ... they do not go from fully closed to fully open at the rated temperature. But once some degrees beyond that temperature either way, they should be fully open or closed. Between those numbers, part open is pretty much what you can expect.
If the thermostat seems "slow" I would check to insure there isn't air in the cooling system.
New-in-box thermostats have been known to be improperly operable enough in my experience that I now check them before I install them.