The best way to avoid a flood is to set the water level in your sump so that in the event of a power failure any water than can be siphoned back down the return line can be maintained in your sump. Secondly popping a small hole at a point (in the tank) just below the water level will let air into the line and break the siphon to stop the reverse flow of water down. To test it kill the power to any return pumps and see where your water levels end up... if it's too close to flooding just turn the pump(s) back on and adjust until it's at a safe level.

Check valves are great when they work.... but Murphy's Law says they will work fine for testing... but they will only fail when you really need them. Two may help this a little, but they do add head pressure to your return line.