We loaned our lawn mower to a friend, since at the time we had somebody doing our lawn for us, due to us not being home during the day for the most part. He recently returned it, and noted that he wanted to be helpful, and had replaced the blade. I took a quick look, and the blade looks like it goes right up against the edges, and doesn't move at all when I apply a little bit of pressure. How can I tell if he installed the correct blade? He did say he still has a receipt for it, so I should be able to return it.
Read the owner's manual - online if you've lost yours. If the blade is new enough to return, read the size off it. If not, use a tape measure on it. Does it match the size called for in the manual?
Depending on the mower, a blade brake my explain not moving, but usually there's a bit of clearance.
Beware of attempting to move the blade if you have not removed the spark plug wire - people do lose hands or feet that way when they manage to start the motor.