Birth decade could get pretty weird when we've got people from all over the place as well, like we'll just end up with outliers from different numbers of centuries ago. Or hell, different numbers of centuries in the future. Internet familiarity seems like a nice solid starter, maybe with like, a thing for them to say if they only got familiar with it since coming here.

[In his experience, he can at least tell when he's not previously skilled in something, so in theory shinki should be able to figure out whether they only learnt about the internet here.]