How can I tell the difference between the Yetzer Hara and the Yetzer Tov?

  The Yetzer Hara is a nag, the voice of the Yetzer Hara keeps coming back and coming back.  But the Yetzer Tov comes softly.  It plants an idea in your head, then it goes away.  Then it flits back, then it goes away again.

from a Shiur by Lazer Brody , "Ancesteral Merit"