Compare the subject material of the art. The church and Christianity were at the center of Medieval thought, and this is directly observable in the art of the period. The Renaissance began to place emphasis more on the independence of man. Consequently, its art became less religious.
Consider the relationship of characters to each other. Renaissance beliefs focused on human qualities and placed them in a comparable position to religious tenets. Michelangelo's Sistine Chapel is the perfect example because it depicts God and Adam being on a parallel level of height. Art of the Middle Ages would have placed any humanity below heavenly figures.
Observe the use of perspective. The Renaissance saw several technical developments in the visual arts, one of which was the use of three dimensions. Medieval art tends to be more two dimensional and flat. Attempts at multidimensional representation in Medieval art lean toward basic up-down relationships: religious figures, and to a lesser degree, royalty, appear above while earthly or common characters sit below.