How can a military combat pilot indicate surrender?
There is no universally accepted means of surrendering an aircraft. According to Humanitarian Policy and Conflict Research at Harvard University,


  Aircrews of a military aircraft wishing to surrender ought to do everything feasible to express clearly their intention to do so. In particular, they ought to communicate their intention on a common radio channel such as a distress frequency. 


They can do other things like rocking the wings, jettisoning the weapons, flashing navigational lights etc. However, none of these are universally accepted or even have a clear meaning in the heat of combat. So the best course of action will be to bail out.

If a pilot is captured by an enemy, there would be no need to surrender.