If the mower is not running and gas is leaking from the carburetor you can be pretty sure that the float/needle valve are not closing off the fuel and the bowl is overflowing.
If the mower has not been running and not properly prepared for storage it will most likely be residue left behind after the gas has evaporated.
Most likely is that the float is stuck to the bottom of the bowl.
Answers & Comments
If the mower is not running and gas is leaking from the carburetor you can be pretty sure that the float/needle valve are not closing off the fuel and the bowl is overflowing.
If the mower has not been running and not properly prepared for storage it will most likely be residue left behind after the gas has evaporated.
Most likely is that the float is stuck to the bottom of the bowl.