Visual's the way to go on the reeds. Check the edges for fraying, splintering, cracking and also check for gaps between them and the cage. A small amount of gap's not too big of an issue, as the pulse will make them seal, but the better they seal w/o pulse pressure, the better.
Main thing to watch for is physical damage.
If there is a gap, and the reeds are in good shape, try flipping them over and see how well they seal.
I wouldn't use the reeds as the sealing for pressure testing - don't think they seal "that" well.
Go to the local hardware store and find the rubber plugs that have a wing nut on them to expand the rubber ring (used for sealing PVC pipe ends) - there's one that will fit right into the exhaust flange and one that will fit right into the intake boot. - may even be the same one.