On the head shown it says PLUMB in the box, which is the typical and most common logo. You might also find though in your searches similar logos such as Plumb Victory, Plumb Warranted, Plumb Guranteed. These are pretty common too. The Plumb Victory is in relation to world war 2 efforts, so they are usually a little older than most, but not always. Also, if you see them with the permabond tops in the eye, where the eye is glues over so the handle can't slip out, those are fairly new, and can range up to the present day. Also common are the Plumb Boy Scout hatchets. You will see the differing logos on one side, then the boy scout seal on he other. Also always remember, Plumb was in its own right a huge maker of axes and hatchets, but also had many "sub-contracts" for other companies, like hardware companies, and would make axes for them as well with their own logo. So make sure you are checking all over the head. On axes Plumb usually offered the head weight somewhere on it, more oftern then other manufacturers.
Also if you ever come across one that says Yerkes and Plumb, those are old, that if from the beginning stages of the Plumb era.
Hope it helps.