Run Prime95 (or Orthos for multi-core CPU's) for about 10 minutes.
If the system is unstable, the errors usually occur within a few minutes.
Once you are satisfied with the OC, run Prime95 for about 1 hour.
If there are no errors, you can be about 99% sure that the system is stable. Instability will otherwise manifest itself in heavy applications, like games. If that happens, try to adjust your CPU, RAM and perhaps NB voltages first, before changing frequencies.
Good luck!