Any skit that includes a rotary phone. I'm thinking of one in which Ernie was awaiting an important phone call from Bert, but Cookie Monster wheedles the telephone out of Ernie to "call his mommy". CM makes what Ernie termed the longest phone call in history. For example, CM recounted to his mother, quite literally the jump rope game (I think that's what it was) he played. He counted "1, 2...," cut to some other sketch; upon returning he continued "One thousand <something>, Four thousand <something>.." After Cookie Monster finally hung up, he "thanked" Ernie by eating the phone. Whereupon the phone rang inside his belly. Ernie held CM's mouth open while he talked to Bert: unlike CM's phone call, we could all hear Bert's tinny voice on the other end. Ernie's last words on camera were "Can you talk a little louder? We have a bad connection."
I think Mr. Hooper's store also had a rotary pay phone.
Any street scene before about 1972 would automatically be dated by the mailbox on the set. During the first 2 or 3 seasons, most mailboxes were painted red on the top section, blue everywhere else, with the words "U.S. MAIL" in white block lettering on the side. The Sesame Street mailbox was painted solid blue at about the same time as the real world counterparts, but with one major difference: instead of the USPS bird symbol on the side, it had a painted "banner" reading "U.S. MAIL".
Any sketch featuring WALK and DONT WALK is now dated, since these are being phased out in favor of hand/man symbols in the US. Most other countries continue to use the red man standing/green man walking symbols instead.
There's this one film where a camera zooms in on a single letter on a sign somewhere in New York City as an unseen kid's voice reads out the letter. Of course the action is done 26 times for each letter in the alphabet, and a sedate, repeating musical score plays in the background. Aside from M for MAIL on the old two-toned letter box, and W for WALK (on a Marbelite pedestrian signal which hasn't been seen in New York City for 20 years) there are probably other anachronisms in that film.