I suppose the first check is always sufficient fuel flow (i.e quantity of gas delivered per minute into an open container).
Release the fuel pump discharge line at the carb inlet, stick a rubber hose on the open end of the gas line and lead the other end of the hose into a coffee can, mason jar, anything as long as it can hold 16 oz.
Crank the engine - it does not need to be running - for 15 seconds while directing the flow of fuel into your container.
If your pump will move at least a pint of gas in the 15 seconds - you have demonstrated good fuel capacity across the entire system (up to but not including the internals of the carburetor). Also, while the cup is filling, keep the end of the hose submerged and watch closely for entrained air bubbles in the gas stream- more on that later.
The next test is pump discharge pressure.
There is an involved procedure for this test that involves cutting into the fuel line between the pump and carb, inserting a tee with a gauge on it and checking the pressure with the engine idling. It is supposed to be about 5-7 psi in case you are interested.
An easier way is to just dead end a gauge on the gas line that is already open for the capacity test. There should be enough fuel in the bowl to get a pressure reading before you run dry and the engine stops - or you can keep it running with ether.
Then there is my way - the hillbilly way; while your assistant is cranking the engine for the 15 second capacity test, play with the stream of gas. Pinch the hose and release it, if it blasts out strongly when you unkink the hose - you're okay. Remember you are only looking for 5 psi. Alternately, you can put your thumb over the end of the hose and play with it like a little kid sprays a garden hose. If the gas can be shot any distance - say only 5-6 feet for while you make your finger nozzle - again you have good enough pressure.
The third thing to check is the condition of the suction piping. This includes the metal lengths as well as any rubber jumpers. Look for kinks, bulging or otherwise distorted sections that could indicate areas where flow restriction might occur. This type of damage will cause an otherwise good pump to appear bad - it is starved for flow.
Sometimes (actually most of the time) suction line problems only become apparent under heavy loads (i.e. periods of high fuel flow).
Closely related to restrictions in the suction piping is air ingress. While inspecting the suction lines look for signs of possible leakage - into the line. These areas will normally be found in piping that is above the normal fuel level within the tank. That was the reason to watch for air bubbles while doing the flow test - but lack of bubbles does not guarantee a tight suction line.
These air leaks are not usually noticeable. While the pump is running, it draws the air in along with the gas from the tank. This entrained air will ultimately end up in the carb and will also reduce pump capacity at high flow rates.
It is conventional wisdom that if there is no gasoline on the driveway - well there isn't any leaks. Not so. If the leaks are located above the level in the tank; when the engine is shut down and the pump stops pumping - the gas is allowed to drain back into the tank and air fills the suction line. No leaks to the driveway but air can still be getting in.
Any ideas on why the bowl is draining back into the line?
As the engine sits idle, the gas in the discharge side of the system (due to head pressure can also back flow if the check valves within the pump are not perfectly tight. As the gas flow back down by gravity, it is displaced by the air that leaked in.
