2 ways:

1. The light shape itself most of the time looks, or has an arrow at the end of one of its' lines showing direction.

2. Select your light, and click the MOVE tool.
Then below the move tool, is the "Show Manipulator Tool".
(hint: with your object selected, just hit Y on your keyboard to invoke it)

This will let you see the light "target" or general aim direction

Then, just move the handles like any other object to re-aim the light aim