basically
males have much more color than females
(but in some strains of guppys females are nicely colored also)
...also
females grow larger than males
...the biggest female guppy I ever saw was 3 inches,
the biggest male I ever saw was 1.75"
...also
males will usually have bigger caudal (tail) fins
and
bigger dorsal (top) fins
than females
...also
males have a visible gonopodium (reproductive organ)
on the belly, females don't have this
http://www.aqualandpetsplus.com/Livebe6.jpg