Connect the scope's leads to the cap leads, and set the scope to the lowest voltage division. Now, you'll need a lamp, radio, clock, anything that runs on 120VAC and is easily portable. Plug it in where you're working, turn it on, and run the cord across the table in front of you. Lay the body of the cap on top of the AC cord, and you'll see the 60Hz AC noise it induces on the scope.

Now, reverse the leads on the cap around the other way. Notice which way has the lowest induced noise (one will have MUCH less). Whichever cap lead is connected to the scope's ground lead in the orientation with the lowest noise is the outside foil.

There may be some other way, but this is the classic method and the only one I know of.

Thermionic