The thumbnail test is what I use. It's probably quite conservative but I know the epoxy is still green.

I agree the thumbnail test is a good one. I have good success with MAS using the thumbnail test. They have been recommending the cotton ball test recently in that this will give a full chemical bond between all layers where the thumbnail only assures a partial chemical bond.

For those unfamiliar with the cotton ball test: Take a cotton ball (I use a Q-tip) and press it into the epoxy. When the resin is too wet the ball will come off cleanly, when the resin is too far cured the ball will come of clean. The "ideal" time is when the epoxy grabs the hairs of the ball and they cling and stick to the boat, in between "wet" and "dry".

At this tacky stage your new coat of resin will not float resin under the glass, but will still be able to react chemically with the majority of the existing epoxy molecules, where as the cure goes farther there are fewer and fewer remaining chemical links available.

For the tolerances we are working at, the thumb-nail hard stage is probably good enough even if it is not "ideal". This isn't a MAS epoxy thing, just a chemistry thing.