I don't think you'd be able to tell all the deatheaters by the Dark mark, surely it is something only the very inner cirlce of Voldy's deatheaters have. When he summons them, he doesnt summon everybvody who ever helped him does he? Just his inner circle.
I am sure somewhere (I am too lazy to find a quote) it says that greyback was allowed to wear Deatheater robes, but wasnt given a dark mark? or did I imagine that?
I am undecided whether it would be visible all the time or not. When the Dark lord was powerful mayvbe it showed and when he was out of power it didnt? I dont know, but yeah, it seems pretty stupid to tatoo all your followers when they're trying to hide that they are your followers...