You can tell the difference by the color of the underside of the
leaves. If they are not silvery, then it is a red maple. You can also
tell by the depth of the sinuses on the leaves- silver maple usually
has much deeper sinuses than red maple. There are also several hybrids
between the 2 trees (acer x freemanii) that have intermediate
characteristics of both parents. The culitvars mentioned by david are
actually the A. x freemanii species and not A. rubrum. Post a picture
of the buds.

Toad