hi manx, i am going to take a stab at answering your question.

while i don't know about japanese maples particularly, i do know a little about plants and trees in general.

so here are some things to remember:
if the plant has gone dormant for the winter, you may not be able to tell if it is viable until spring

sometimes (frequently) a plant may appear dead, but the roots are viable and healthy and the plant is just waiting for the best opportunity to grow.

When checking the viability of a plant, the green will only be just under the bark, not all the way through.

but, if you really want to check the viability of a plant, take a sharp knife or pruners and snip just below then end of one of the outermost branches. if you see any green under the bark, then that branch is getting nutrients and the plant is alive.

if you don't, continue snipping on then same branch/twig in 1/2 in increments until you hit green, or that branch is gone.

just remember the above statements when doing this.