Feel the top of the memory foam mattress. Press your hand into it or sit on it for a basic idea of the firmness. If the memory foam hardly moves, it's firm. If the memory foam sinks a little bit, it's medium.
Locate the tag on your memory foam mattress and look for the density of the mattress. It will be in the form of pounds of density.
Check the number of pounds of density to see how firm your mattress is. Five lbs. of density or higher is a firm mattress, while 4 lbs. of density is medium, according to the website Healthy Foundation.