Should be stamped on the monoblock, although I believe some aren't. Even if it were stamped, I wouldn't trust that alone, anyone with a harbor freight stamp set and a hammer could fake that. I should come with paperwork to verify, barring that, if you contact tom directly with the make model and serial #, he should be able to pull up records of what was done.
Why do you care? Many people on this list say it doesn't matter as long as you have so much choke that the pattern is 4" at 40 yards. More choke, more smoke.
Kidding aside, he's done 3 guns for me. When I sold the 2 I was able to include a receipt, plus the barrel was stamped. I suspect that he could also confirm the work if you contacted him with the make & serial number of the gun.
He does beautiful work & is a great guy to work with.