1. Dark Mark
The accused Death Eaters could claim they were under the imperius curse when the Dark Mark was given to them. It is also possible to be a reformed Death Eater; you strongly regret ever turning to Voldemort, and now only help the good side. If determining if someone was a Death Eater was as easy as just checking their arm, then these good people (think Sirius' brother) would be grouped together with all current Death Eaters.
Harry could have tried rolling up Draco's sleeve, but getting close to Draco and then tricking him to roll up his sleeve is no easy task. Even if Harry manages to check his arm, Draco would inevitably be aware of this, and Harry probably wouldn't want Draco to know about his suspicions.
2. Veritaserum
Harry Potter Wiki on resistance and limitations of veritaserum:
'Despite being the most powerful truth serum in existence, it can still be resisted through different methods, including the taking its antidote and through Occlumency. For the same reasons Muggles use polygraph tests, Veritaserum is no more reliable than its Muggle counterpart. Since some wizards and witches can resist its effects while others cannot, Veritaserum is "unfair and unreliable to use at a trial" and cannot be used as definite proof of guilt or innocence.
Another weakness is that the victim only states what they believe to be true, so the victim's sanity and perception of reality also factors in during interrogations. This is the main reason why Barty Crouch Jr.'s testimony was not credible, as he was clearly insane.'
http://harrypotter.wikia.com/wiki/Veritaserum
3. Blood status
Blood status doesn't just depend on your parents, but also on your grandparents. In order to be a pureblood, you need two sets of magical grandparents. As we know, Harry's maternal grandparents were both muggles, and so Harry is not a pureblood. Because his paternal grandparents were witch and wizard, Harry is a half-blood.