I usually don't say anything unless asked. I've had a run in or two with dealers misrepresenting the grade of Darne guns at shows, but, that was more of a problem before Al Gore junior invented the internet. If you get taken now, shame on you, that information is out there.
A long time ago, I did ask a dealer about a Baker I was interested in, as to whether it was an "A" or an "R", as I didn't know enough about them to know for sure. He told me he didn't give a crap (not the exact word he used) about any letters on the gun, just the numbers on the price tag.
He was a dink, but, taught me a valuable lesson that day. Don't buy what you don't know.
My forays into expensive guns have been brief, and some have ended badly. The only guns I will ever make money on, I made that money at the time of the buy, not when it eventually sells. I don't really collect guns, and have them to use and enjoy. None of them will look new when they are sold.
I have a model 1912, first year 20 gauge, that most would say is badly refinished. It has new wood, and was re blued, without a lot of concern for the lettering on the barrel, as it was worn off in spots from use and abuse. But, I use it, and enjoy it just the same. I have had guys drool on it, asking about the great condition, and I always gently point out it was a roach, that has been refinished, and show them how to tell.
But, that is my gun, and doing that is different than slamming their's.
I buy so little these days, it isn't worth making a huge deal out of someone else's gun to me.