If there was a way to tell, I think it would be by looking inside the tube at the closed end. I think the 6061 tubes have end caps which are welded in place. I think the 7075 tubes are drilled/machined (but not all the way through). If so, the inside ends of the tubes should look different to the trained eye if you could look closely enough.
I hope someone with more information posts here or clarifies anything that I have said.
Joe Mamma
"Reliability above all else"
NRA Certified Pistol and Rifle Instructor, Life Member
Glock Certified Armorer
Beretta & Sig Sauer Certified Pistol Armorer
Colt Certified 1911 & AR-15/M16/M4 Law Enforcement Armorer
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