The easiest way is to use the CE information screen. This has the advantage of placing extra stress on the CPU.
If you
-set governor to performance (there is a CE setting for that)
-play a movie and open the information panel you can observe the temp and CPU clock in real time.
If it sticks at 1.9ghz then you do not have throttling issues, you should also observe that the thermal controller and throttling kicks in at the trigger temp of 70C (or close to), but these on chip tempo sensors are not that reliable.
You only need to see this for a few minuites to confirm if you have an issue with overheating. Setting the governor back to ondemand might then be all you need to do to avoid throttling - but there is no real way of checking this.
Shoog