You can look it up in Bruce Hoadley's "Understanding Wood", but there's
not all that much "hardness" difference between the two; but hard maple
is slightly harder and denser and as the posts indicate you can judge
by heft. The way I tell the difference is that on a fresh cut or
planed piece that is not the end grain, the soft maple has a slight
greyish tinge to the wood, where the hard maple is a brisk white. I
made duplicates of my kitchen cabinets for the laundry room, and used
hard maple. When compared with the soft maple of the commercially made
units, you can tell the difference right away, hard is much brighter in
color. For cutting boards soft will be just fine.

Mutt