#like 'why didnt they use veritaserum on sirius!!!' bro didnt even get a trial.

[seriousbrat]

How come do you think Snape never informed other order members about the Dark Mark? When Harry brings up to Sirius in GoF that Snape and Karkaroff were showing their arms to each other, Sirius has no idea what it means. Surely once Snape defected he could have made the Order aware that an easy way to spot a Death Eater is to check if they have a big spooky tattoo on their arm? Do you think he was embarrassed? Or he told Dumbledore and Dumbledore chose not to tell the rest? Is this another JKR hole that will leave us doing gymnastics to get around?

I believe it's implied that the Dark Marks basically vanished after Voldemort's downfall in 1981, and only started coming back during GoF, so it wouldn't have been a reliable way of identifying Death Eaters after the war. Snape only switched sides at the very end of the war, and Sirius and the Potters all went into hiding so I just don't think this information would be of use or relevant to them.

The other thing is that they likely didn't need it-- I'm sure the Order was aware of who many of the Death Eaters were, even the ones who escaped Azkaban at the end of the war, without having to check people for tattoos. Sirius lists Death Eaters like Lucius Malfoy and Avery who claimed they were under the influence of the imperius curse.

So Snape probably did tell Dumbledore about the Dark Marks, it's just that it wasn't particularly useful information at the time for the rest of the Order and especially not once Voldemort was defeated and the marks disappeared. The Ministry was in utter chaos at the end of the first war too, so even if Dumbledore did provide them with info about the Dark Marks they were already desperately scrambling to catch the Death Eaters they already knew about and I don't think it would have been of much use. That's not even considering that there are probably magical methods to conceal tattoos, since there are certainly muggle methods that are pretty effective. So I don't think it's a plot hole, no.