Penny test is done to detect:
Test for Checking Unsafe Illumination:
Film fogging can occur due to:

Use of inappropriate safelight filters.
Excessive exposure to safelight filters.
Stray light.
Coin test/Penny test (A and B) is a test to evaluate for fogging caused by inappropriate safe lighting conditions.
	
Shut all lights and put on the safelights.
Open the unexposed film packet and place the base film in the area where films are usually unwrapped.
Place a coin on the film and leave it in this position for approximately the time required to unwrap and mount a full mouth set of radiographs, usually 5 min.
Solution contamination
Defect in exposure
Unsafe illumination
Density of water