I assuming you are trying to find the draft (shoal draft or regular keel) rather than material and the shoal draft has a bulb and the regular draft is a fin keel of some sort.
If there is a bulb on the shoal draft keel for this a Newport 31 and you can get in a dingy or are in a slip. Simply get a long pole (with a squared off end if possible like a piece of pvc pipe) and push it firmly along the keel and down. If you have a bulb at the bottom of the keel it should "hang up" a little or even completely stop when it gets to the bulb. If its a fin keel it should just keep on going without hanging up. A lot cheaper than a "go pro" (but not as much fun) or a diver.
Of course I'm assuming it is the "type of keel" you are inquiring about not the material?