Sure, but you have to take it out.  It will take you ten minutes to replace
the thing... just replace it and see if the problem is fixed.
--scott

Sure.
1.  Check the temperature gauge.
2.  Immerse the thermostat in a pan of boiling water and check the
temp with a candy or similar thermometer.

On lots of vehicles you can open the rad cap on a cold off engine, then
start it up and look inside.  If you have fluid flow right away, your
thermostat has failed.

I have seen an epidemic of failed open thermostats in the last couple
years, mine and our tenants' included.  More than I have seen in the
last 40 years.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
'New' frame in the works for '08.  Some Canadian Bush Trip and Build
Photos: http://mikeromainjeeptrips.shutterfly.com

Idle the engine from cold start.  Feel the front center of the radiator
with your hand.  Stay away from the fan side, because electric fans will
start whenever they feel like it.  If the center of the radiator warms
up early, well before the gauge reaches the set point (85-95 C), then
your thermostat is stuck open.

I also like Mike's suggestion to look for flow in the top of the
radiator.  IMO sometimes that's not enough because of radiator geometry
or thermostat stuck partially open.  So do both; more info is generally
better when working on cars.

Oh yeah, you might have a bad temp gauge, but the chance of a bad
thermostat is a lot greater.

IIRC all modern car thermostats are at least 180F = 82C, so from your
gauge symptom alone, it is most likely the thermostat.  Does the heater
seem weak too, never doing a good job warming up the passenger
compartment?  That is more evidence for a bad thermostat.  If you know
you have a weak heater too, then more testing is probably not even
necessary.

--Dave