Is there any convention on how to mark an all metal hotend to distinguish it from a hotend with a PTFE tube in the heatbreak? Once assembled for a German Riprap 3D printer, they look exactly the same on the outside. I'm trying to decide on a way of marking them to indicate the difference.
Given your heatbreaks are the same material and external dimensions, it seems that the most practical way to distinguish one from the other would be to mount the all metal heatbreaks in a heat sink that's anodized to some other color than natural aluminum -- red or blue, for instance.
If you can't buy them that way, or already have plenty on hand, anodizing is fairly easy to do at home -- you need battery acid, a car battery (or heavy duty charger for one), and some strongly colored dye to apply after the anodize has established the oxide layer on the aluminum part. There are many online references on how to proceed and how to stay safe while handling the acid.
If you have machining capability, another option would be to make, say, a copper heat sink for the all metal heatbreak, while keeping the aluminum for the conventional one.
Usually, the neck of an all-metal heat break would be thinner (as it does not need to contain the PTFE tube, and making it overly thick would conduct heat unneccesarily). This may be observable when one loses track of what is where. If one needs to see at a glance, I do not think there is a convention.