Scratch test

From BonsaiWIKI

The scratch test is a useful way to tell if your bonsai is still alive. If your tree has no leaves or has brown leaves or needles on it, you may be wondering if it's still alive.

Using a fingernail, scratch the bark of the tree and see if you can find any green cambium underneath. You may need to scratch fairly deeply. At least 1/16 or so. If it's green, that's very good news. Your bonsai is still alive at that point. If not, and you see brown or white, it means that that portion of the tree is probably dead. Try scratching down lower, closer to the soil line.

Note: this method doesn't work very well for trees with rough bark, and non-broadleaf species like needle evergreens.