My personal rules of thumb- first few years prune EVERY clem down to 2-3 nodes, if you not sure wait a little until you can kind of compare and get a feel which node is looking like having life and which does not.
After clem has at least 6 stems coming out I prune hard those that bloom late and easy/ just minor cosmetical/ those that bloom early. I leave alone anything that belongs to group 1/ blooms the earliest for me/ and especially tough large stems.

A viable leaf node is essentially just a swelling bud that you can easily see on any stem. A dead bud will remain brown and won't swell in size. Most times the plants will actually start showing green on the buds which is a dead giveaway that the node is viable.