It's hard to tell, actually, and the hue is even harder to capture in photographs (even shot in natural light, which is the most "honest" lighting).
There are a few variables, however. Steel & white gold have more of a cooler, grey look to them (white gold being a little less cool than steel).
Generally, platinum has a warmer look, like that of pure silver (it has a bit of white-ish hue to it):
(platinum)
Occasionally platinum has a palladium plating, but this is not common; platinum is usually just highly polished, like steel.
But white-gold isn't a periodic element, like gold or platinum; it's an alloy It's not actually "white", per se, it's more yellow-ish in color, and is generally coated with another alloy--either rhodium or palladium:
(white gold)
This is why "pure" white gold shows wear & tear very easily; it has to be replated every-so-often.
Steel has a darker hue than the other two:
(steel)
But it depends on which type of steel. The sides of a Submariner case, for instance, are merely highly polished, not plated:
(polished steel)
The bezel of a steel Daytona, however, is also polished and then rhodium plated:
(rhodium plated steel)
(all the watches pictured are genuine, and are shot in natural light)