What's a potential turning point?  Think of it this way:  They're the moments when someone is one panel away from saying, "What the ---!"  Some classics are:

When the hero gets knocked out.

When the vllain is about to be captured.

When the superweapon is fired for the first time.

When the evil mutants destroy something.

When the hero is out on a date.

When the battle goes over a cliff.

When powerful radiation is unleashed.

When there's a press conference going on.

When the flying car goes out of control.
 

And perhaps most importantly:
When things get dull.

Dull moments can be the number one killer of roleplaying fun.  If things get dull, you have many choices to make.  Create a new turning point, or....find a way to bring another hero, or villain into the mix.  Eventually, its up to the players to decide when its TOO dull.