What are the physical differences of a 7075 T0 and 7075 T6? Can you differentiate the two by looking at signs like water marks as a result of quenching or other means?
The main ones are going to be strength and hardness. The best option is going to be to take a sample and tension test it. From working with O temper (I’m assuming that’s what you meant by T0) in the past, it is much more susceptible to surface damage than T6, so I would think hardness testing may be able to help differentiate it, but I don’t know what hardness ranges you’d be looking for or how reliable it would be as an identification method. Also, I wouldn’t rely on looking for any surface markings or anything like that since it’s possible there’s been a post-quench operation to alter the surface finish, which would erase any hint of quenching that may have been there (and I wouldn’t even rely on looking for water marks in the first place). I’ll also say that an experienced metallurgist probably has more tricks up their sleeve to differentiate them than I’m aware of (I’m just a mechanical engineer).
Gat,
If it matters to you, consider specifying something like Vickers hardness on your drawing. This puts your fabricator on notice that you are paying attention. Vickers hardness testers are fairly cheap, and probably fairly available from sub-contractors in your area.
Why do you think you have 0 temper material? My understanding is that soft material is difficult to machine. A machine shop might substitute 6061-T6 for 7075-T6. This is still detectable by hardness tester.
I got into this discussion because we had a 6061-T6 part that had been brazed, and we were wondering what state it was in. We had access to a Brinnel tester, and the material turned out to be annealed.
No one is just an ME. That is a worthy trade. 
At first I thought you had two different alloy numbers. By definition I would think hardness is the only difference.
Rev did heat from brazing anneal the whole part?
Aramis,
I assume it did. My thinking was that the part had been brazed, therefore, it is no longer heat treated. We tested to verify that.