Identify the radiating edge of a multifaced patch antenna

How can I identify which edge of the antenna is radiating for a particular resonant frequency, especially when multiple edges are there for a microstrip patch antenna?

Is there any condition that need to be satisfied to decide which edge is supposed to radiate with respect to the position of the input feed?

What information do you have available? You should always be able to tell by observing the fields; if you only have the far-field data, you may be able to infer by comparing different field components.

The input feed should be impedance matched for the particular radiating mode; for example usually you want to feed a rectangular patch on it radiating edge -- it's much more difficult to match to the orthogonal mode.