Calibrating the oscillator of a microcontroller
If you are willing to do the measurement over a period of time (one midnight to the next), the frequency of the US power grid is maintained to exactly 60 Hz over a 24-hour period as discussed here. The frequency will vary during the day, but it is trimmed at the end of the day so that exactly 5,184,000 (60*60*60*24) cycles will occur from one midnight to the next. 

To count this, you will need a zero-crossing detector.  There are many such circuits on the web, here's a fairly simple one:



Count up how many cycles your clock is putting out in 24 hours (perhaps after going through a divider), and then compare those counts with the counts off the power lines.

Run this the first time and it will give you the frequency.  Run it another day and it will give you the drift.