It's pretty hard to miss. :D
Even the A/C cars had them, at least, my '71 Dart does. Just look up. If there's a big round hole with nothing over it, the vent box is missing.
The vent box shouldn't keep anything from leaking per se though, the lower cowl has a lip on it so water should go around it and into the drains, not into the vent box. Of course, if the lower cowl is full of crud you might get a significant amount of water in there.
