A more definitive test can be done by inserting a Phillips screwdriver (with heavy insulated plastic handle) into the spark plug boot, then hold the metal shank near (1/4" or so) a metal surface or bolt on the engine while someone cranks the engine.
I have seen some "spark tester" lights that will illuminate even when a spark too weak to fire a plug comes down the wire.
Using a spark tester where the actual spark is observed (instead of a light) is much more revealing about how robust the spark energy is. The screwdriver is the poor boy open spark test method.