The easiest way to tell the difference is the area where the timing pin is inserted into the cover. On a Model A timing gear cover the raised area surrounding the hole is round and on a Model B cover, the raised area is oval shaped reflecting the initial advance of the Model B ignition. Once you know that difference, the other timing cover that you might encounter is that of the '33-'34 fours. The raised area around the timing pin hole reverted to a circle, but the generator mounting location is noticeable set back and the spacing of the motor mount bracket attachment holes along the bottom of the front of the cover have a very unique spacing compared to either the A or B covers.
I've added an out-of-focus photo of a Model B cover where you can at least perceive the oval area around the hole for the timing pin and a somewhat better photo of the very different '33-'34 timing gear cover. Sorry about the photo quality.