In my fantasy world, criminals who have committed serious crimes are sent to an island called "Yaehlbuhn". This island, quite far from any continents, is essentially an open prison where criminals can live in almost total freedom.
Law enforcement is present but only act when large conflicts breaks out. They are pretty brutal and don't really care about bystanders and such, after all, everyone on this island is supposed to be a criminal unless proven otherwise.
Enough exposition, here is my question.
I need a reliable way to identify people who were sent to the Yaehlbuhn or live there. The general idea would be that, in case someone were to escape, anyone would be able to tell that this person comes from the island.
I thought about tattoos at first, but depending on the size, you can hide it pretty easily, you could even remove your skin to erase it permanently. (If you are really determined.)
I would also need to find a way to identify people who are on Yaehlbuhn for lawful reasons. (such as Guards, officials or any other visitors) As you can imagine, it should not be easy to reproduce.
The government has access to pretty much anything, but low-cost/low-tech (and cool) options are appreciated.