As mentioned by others, the silver blade test. However, we haven’t seen Samuel touched by silver, yet. All we have to go on is Sam’s word that Samuel is Samuel, and that the Campbells are really the Campbells. (Or if we did see the silver test, I missed it.)
1973, actually.
I’m not as embarrassed as I probably should be that I know that off the top of my head. It was a really good episode!
Oh, I know WE didn’t see it, I just assumed that Sam must’ve done the test at some point prior to the season premiere.
How about testing Sammy while we’re at it? I don’t think that’s the real Sam, either - Dean’s point about Sam luring the Alpha shapeshifter to come kill all the hunters was not a bad one (but I’m not convinced Samuel is who he says he is, either). Something’s definitely got Dean’s spider sense tingling, and he should know enough to listen to that by now.
Sam already proved he was Sam in front of Dean in the first episode.
Also that fact that the actor who plays Grandpa Sam has had major work done is totally distracting. He sorta reminds me of a male Joan Rivers, too much manscaping! Campbells totally hinky, Sam super hinky. Tired of Dean always getting the crap end of the stick.
Wait. Skinner from X-files has had work done? He looks way older than he did on X-files, so I didn’t notice.
Refresh my memory - I’m not recalling that. 
From TV Without Pity