So there is a species of shapeshifters who use a tattoo in a specific location on their bodies as a "belonging" indicator. Basically it is a symbol of group pride: it gives information about what the person's extended family and national groups are. The shapeshifters therefore like to show it off and will usually leave that portion of their body uncovered when they are in a form of similar shape and structure to their natural one (which isn't uncommon, since the closer you are to your "real" form, the more comfortable it is and the less energy it takes to maintain.)
How the shapeshifters generally work: these are shapeshifters with limited options, not "I'm a living puddle of goo and can turn into a vase" shapeshifters (looking at you, Odo). They (generally speaking) can't add or remove mass from themselves, just expand/compress it to a limited extent and reorganize it. They also can't turn into anything that isn't alive, and usually are limited to things that actually exist (and that they have personally seen/interacted with). At the moment the hand-wavy concept behind their shapeshifting is that there is a special substance in their blood that "fuels" (provides the energy for) the shapeshifting, and most of the cells in their body are stem-cell-like in that they can be turned into any other kind of cell at will (and can be multiplied rapidly if necessary). This also allows them some pretty enviable healing/regeneration speeds. As previously stated, they have a "natural" form, which is reverted to upon death or when the shapeshifter for whatever reason stops trying to maintain the altered form.
Obviously, there is a bit of conflict here, hence my question: how do you give people who can reorganize and regenerate the cells in their bodies a fairly permanent "tattoo"?
Mind you, I'm not asking how to make the tattoo visible in every form the person in in. I don't care if it disappears when they change into a dog or whatever. It might be covered by fur, or just gone, whatever. My primary concerns are these:
With the amount of cellular reconstruction and regeneration that goes on with this species, a human-style ink tattoo might fade faster than normal, or even disappear, and I want this tattoo to be pretty permanent.
Also, I don't want this thing to be super simple to fake. For instance, you could probably just "make" a "tattoo" using shapeshifting, but then you can rearrange the shapes and colors at will and pretend to be somebody you are not, and while this would probably be an extremely taboo thing to do, I would prefer if it wasn't that EASY. It doesn't have to be impossible, but not stupidly easy.
The only halfway realistic thought I've had was to use some kind of DNA tagging/imprinting that programs things so the tattoo will naturally reappear as cells refresh, but that is kind of high tech for something that is supposed to be a very old cultural tradition, so I'd rather find an alternative solution, if there is one.