"I believe the rule of thumb is that the front hub should be hidden by the handlebar when you look down at it."


exactly - if it isn't you need to adjust. If it the hub is just partially peeking out you could probably just swap in a longer/shorter stem. if the whole hub is visible, you could be riding a frame that is just too long or short for you.

Of course there are other variables, such as seat-bar height releationships etc. Bottom line, if a frame is the wrong size you may not realize it but once you can compare it to one that is the right size, the difference becomes more obvious.