Just take a knife and carve out that 'overhang' on the transmission case. Then you can see the mesh. I'm not sure what value that overhang has anyway - since a pebble would never get stuck in there given the rotation direction of the two gears. So it's not to keep debris out.

But yeah I also do the 'feel' test without paper. I aim for the smallest amount of play I can achieve, but not zero. Make sure when you check that you rotate the entire spur gear at least one full rotation to make sure it doesn't bind anywhere. And do those engine bolts up tight - if you hit a bump and the engine moves, then bye-bye spur!