Any easy test method is dropping a dye tablet into
the tank. If the flush valve is leaking, the dye appears
in the toilet bowl after several minutes. Dye tablets for
this purpose are a standard hardware store item, but any
dye can be used that does not stain the toilet. Food
coloring works well.
Another foolproof method is to turn off the water
supply to the tank when it is full. A leaking flush valve
allows the tank to drain. You can usually tell that the
water level has dropped within a fraction of an hour. A
bonus of this method is that it exercises the shutoff valve,
which typically freezes from lack of use.