While these are not definitive tests, they can help.
First off is the thumb nail test, if you can easily dent the wood with your thumb nail, it probably red maple. If your tumb nail glides across the surface leaving a kind of shiney groove is probably sugar maple.
Next the gouge test, take a sharp gouge and cut cross grain. If the cut is well defined and has a shiney quailty to it is probably sugar maple (or a harder piece of red maple) If the cut is kind of fuzz, the wood crumples, etc. its probably red maple.
And finally the color, sugar maple tends to have a pinkish hue to the wood. While red maple has more of a white/creammy tone.
Like I said one of these are definitive, hopefully they help.