Your conscious will let you know.
How do you know if you are doing the right thing ?
I don't. Only time will tell.
As others have said, you should follow your conscience on this. If you're looking for a more practical way to break a situation down - to analyze things - just to be sure, try the following link:
Well the Right thing is on the Right, while the Left thing is on the Left . . . If you were doing the "Left Thing" you would know it by your orientation . . . Chances are if your not doing this one, you were doing the "Right Thing".
On a more serious note, we use Reason, Empathy, and Reciprocity to determine right from wrong . . . You are most likely (unless you are a Psychopath) able to determine simple right and wrong in this manner.