Just thought I'd share my test findings. Feel free to hang crap on my conclusions if you want
Just finished testing the thermostat on my GPX 250 as I had suspicions it wasn't opening.
I bought an Automotive DMM with inductive pick-up for this job which made it really easy as it measures temperature as well.
Once the bike had warmed up I measured the temp of the thermostat housing which was reading about 76 degrees Celsius or 168.8 Fahrenheit when the thermofan kicked in.
The workshop manual acceptable temperature range for thermostat to open is 177 -182 Fahrenheit so I figure the thermostat is still working well.
I can thank the multi meter with temperature probes for helping with this job as I didn't have to pull the thermostat out of the radiator to check the temperature so purchasing one is a must if your doing any maintenance on your bike as it reduces your workload if your lazy like me.
Of course if the thermostat housing is giving out readings well above 182 Fahrenheit then you've started to cook the engine....