Hi MacCullum, you can buy a gadget from various auto shops that is made for doing the spark test, but there isn't much technology involved, and the home-built product is always more fun while suiting your own preferences. Here is mine:
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It consists of an old car spark plug that had earned honourable retirement, gapped to 0.060"; a 14 mm nut; and a 4 for $2 Taiwanese spring clamp. It is best to use a real spark plug rather than a tester based on a fake spark plug, because you can't adjust the gap on the fake ones, and the gap is too big for general use on lawnmowers (it's intended for cars with high energy ignition, and is over 0.100" gap). The tester Briggs and Stratton supply has a fixed gap of 0.166", and is even less suitable for general testing (but it is rather cute):
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If you make your own tester, remember that spark plugs have a 14mm diameter x 1.25mm pitch thread. Common or garden 14mm nuts have a 1.5mm pitch thread (metric coarse) or 1.0mm pitch thread (metric fine). You can fix that by installing the plug into the nut with a fair sized spanner, since it will never be removed and doesn't need to hold any load anyway. Alternatively you can find the right nut. I cheated: I bought a 14x1.25 tap and reworked the nut, but the big spanner solution is quicker and equally effective if you don't mind ruining the spark plug (you could weld it to the clamp if you wanted to, and eliminate the nut. I think most people use a bent piece of tin around the metal body of the spark plug and screw the piece of tin to a clamp, to avoid both the welding and the nut issues.)

With the simple-style tester you just attach the spring clamp to a metal part of the engine, and attach the spark plug lead to the tester's spark plug in the usual way. Remove the ordinary spark plug from the engine so it will spin easily when you pull the starter.

Incidentally at least some of the GV series engines had electronic ignition, so if your spark is erratic you may be up for a new ignition module. However, first test the spark. If it is defective, you will need to do a couple of standard service checks to see that the module is properly grounded, and it has the right resistance values.