Since the blades are not marked how do I know which side of the blade faces the mower and which side faces the ground?

Hi,
The answer will, I hope, sound obvious once said! The purpose of the curved edge, called the "lift," is to stand the grass up for cutting so they go up toward the deck shell. Otherwise they would blow the grass downward.
Peace,
Clarke

In addition to what crigby said, if you were to manually spin the blades in the correct direction to get grass to go out the chute (usually clockwise as viewed from above), the cutting edge should be on the front of the blade, not the back (obviously).

Plus, if you buy genuine MTD OEM replacement blades (like from Lowe's, Home Depot, Tractor Supply, etc), if you look closely you'll usually see the word "bottom" or "down" is actually stamped onto the blades on the appropriate side.