1987 300E Thermostat stuck open

Hi


I have a funny feeling that my thermostat is stuck open and was wondering
if 
there is any simple test to determine if it is open?

I just drove home about 35 miles doing 70 mph on the freeway.  The gauge 
never got above 60C, the weather here is 52F = 11C


I would expect the gauge to go up to at least 80 or 90C

Does the 197 300E run this cold or is the thermostat stuck open and the
car 
never heats up?

Thanks