I looks to me (roughly) like if the signal decays as time goes to infinity then you use the energy technique (integrate the square of the curve) and if the signal doesn't decay then you use the power technique (average of the signal over a time period.) See here. Graphing is probably the fastest check, though it will not always be obvious if the integral of the square of the signal is infinite, you ought to be able to tell in most cases by simply looking.

-Dan