GPU-Z is your friend, set it to log the sensor output and game away.
get an app called Generic Log Viewer to view the log in graph form.
Apart from being able to monitor the clock speed, temps etc over a longer period of time, there's a flag called Perfcap Reason which will tell you, if the card is throttling and why.
Here's an example that I sent to someone over the weekend. In Green is a well set up card, steady, high clock speed, no throttle triggers
in Red is a badly set up card, constantly bouncing between voltage, temperature and power limits.
To Translate
16 is ideal
8 is voltage limited
2 is temperature limited
1 is power limited
4 is probably the worst since that is "the core is not stable at this speed at this voltage" and you're likely to have crashes.
As you can see above, 13 is quite common on this trace...that basically means that it's throttling for a number of reasons

, unstable, power and voltage