I use to not look at both sides of the nickel - especially if they appeared "new-ish." I would date check obverse or look for the distinctive mint mark on the reverse depending on how the coin landed in my hand. If the coin appeared old, I would date check it. Then, like OP, I came across a 1939 in similar condition. I decided to start checking both sides. Out of habit, I still find myself only checking the side facing up, but I'm slowly retraining myself.