Agar is a medium, so you are checking the sterility of the agar. After preparation one usually places an agar plate at room temperature and another agar plate at 35 to 37 degrees C. After 24-48 hrs of incubation a visual check is made to see if there is any visible growth on the uninoculated plates.
If you are adding blood, etc. to the agar, those components can be checked by subbing them to a blood agar plate to see if there is any growth--which would indicate non-sterile components.