. . . into shorter run times? ISTM there were far fewer commercials in the '70s and '80s and shows had longer run times.
Inquiring mind(s) on a Love Boat and Policewoman binge want to know. So far as I can tell opening/closing credits and scenes aren’t trimmed, but there are the usual million commercials jammed into the hour.
FYI: Angie is undercover as an exotic dancer in this episode – ROWRR! 
The most reliable way is to trim scenes. It’s really easy to spot in many older sitcoms because of the setup-punchline format, then you realize someone cut out the setup and there’s a punchline with no context. You say you couldn’t tell that scenes were trimmed, but I’ll bet if you compared the shows minute-by-minute with the originals, you’d find dialog dropped, reaction shots eliminated, etc.
There’s also a technological trick called time compression. I cut the time of a recorded speech by more than 10% by simply compressing the speaker’s pauses (between words and sentences) by 50%.
Sadly, The Andy Griffith Show has really suffered this treatment. Some of the jokes have been really butchered or just eliminated.
Sometimes it’s sped-up, to where you can hear a difference in the voices.
Stings (e.g.: the final scene in any “MAS*H” episode) are often eliminated.
They also play the final credits squished into a box on the bottom of the screen while the opening credits to the next episode is playing above.
Thank God for the DVR!
You see the same thing even on newer shows, once they reach syndication. I know every Seinfeld like the back of my hand, but sometimes when I watch an episode on Crackle I’ll hear a line or two that I didn’t remember, and then I’ll realize it’s because I’m so used to the trimmed version. It’s like hearing whole new material! 
I was watching my favorite Airwolf episode on MeTV (I think) and about 10 minutes in, I noticed a set up for a scene coming later was missing., So I got up off the couch, got the DVD, started watching from the beginning (missing scene returned!) and still got done at the same time.
Hogan’s Heroes are butchered as well. I try to only watch them on DVD, but sometimes at 9:00 you’re too lazy to go put a disc in.
Law and Order puts the credits over the last scene, and speeds up the theme music by a lot. I call it the disco version.
I’ve also seen where the closing credits are run much faster than on the original broadcast.
I was the same, except I saw most of them in the original run, so I would occasionally spot a dropped sentence. The one that comes to mind is Elaine complaining about going to Hell because of David Puddy. The line dropped in syndication was “…and the heat, my God, the heat!”
Dennis
Any throwaway scene that doesn’t advance the plot is likely to be dropped. There was a great one in ***Cheers! ***where Rebecca actually held a lit cigarette inside her mouth to keep Sam from finding out she’d started smoking again. I saw it when the episode was originally aired, and never since.
In the abovementioned Hogan’s Heroes, there was a scene in the episode with the heavy water where Carter tasted a sample to see if it really was water. Again, I saw it when it was first aired, and never since.
These are the first two examples that came to my mind. Star Trek was, of course, notorious for this sort of thing when it first went into syndication. Back then, it was film cutters at local TV stations who did the deed. The clips they were throwing into the trash sometimes got recycled as slides rescued by fans (usually young females who had ingratiated themselves with the film cutters). The clips were often sold on a clandestine market and started turning up at Star Trek conventions in the early '70s. (Remember, this was **long **before there was an Internet!)
My candidate for the best dropped line comes from the penultimate episode of Frasier. There’s a crisis at Martin and Ronnie’s wedding, and an ice sculpture by “the famous artist Nudzac” is in danger of melting:
FRASIER: (Panicking) My Nudzac is dripping already!
ROZ: (Calmly) Relax, it’s a dark suit. It’s hardly noticeable.
Heard this once, never again.
You stole my jesus fish, didntcha?!?
They can also speed everything up but pitch down the sound. There are even side-by-side videos on YouTube. People just do things faster.
Here’s an example from Seinfeld.
Personally, on laugh track sitcoms, I wouldn’t be surprised if they sped up or shortened the laughter as an additional effect.
On One Day at a Time reruns on Logo, They speed up the theme song. It sounds terrible. They also cut the sax of the theme repeat in favor of running the theme of the next episode up in a compressed space over the HIGHLY compressed credits of the next episode. They probably gain three minutes. They also cut some scenes. Usually I miss them-- occasionally not. All in all, they probably compress a little too, and get about five minutes out of each episode. They are better than the butcher jobs METV did a few years ago. They’d leave out whole scenes that explained the motives of the characters in the next big (I mean extended) scene. Logo does have a way of putting commercials in weird places. I wonder if by makes the breaks shorter (and having more of them) they can charge more for the time. ie, “People will sit through it if they know it’s only 1 or 2 commercials.”
In the episode of WKRP where Johnny tells Herb that Jennifer is post-op trans, they cut out that bit, making a large chunk of the rest of the episode very bizarre.
The pilot of The Big Bang Theory has Leonard and Sheldon at a sperm bank. Also cut in reruns. Cutting Althea on TBBT is a criminal offense.
Wow! Fascinating replies! We didn’t have a TV until I was 16, so I wouldn’t necessarily notice if older shows had dropped a scene/line/were sped up. I’ll need to eagle eye these things now.
There is so much expository in older shows that I can see where dialogue/establishing shots could certainly be cut for today’s viewer. I was thinking about the vast differences between series like The Wire and Breaking Bad, which depend on the viewer “filling in the gaps,” wherein earlier TV would have spent a lot time laying out explanations for viewers.
WKRP is almost bad enough for it’s own thread, though the cuts are mostly for music rights rather than time. The episodes were butchered for the very first syndication package, and even the most recent DVD set still has some stuff missing from the first run. With the music changes, some scenes don’t make any sense at all.
“Hold me closer, fancy dresser.”
“Do I hear dogs?” “I do.”
I knew lines were cut from the scenes in TNG where Tasha Yar dies in Sickbay and Picard acts as advocate at the hearing to determine Data’s status the second or third time I saw those episodes. I should watch the show more often nowadays to see if they’ve been restored.
In the original episode of Cheers! where Woody’s parents want him to come back to Indiana, there was a scene after Diane’s film ***Man-Child in Beantown ***where we saw him taking the phone call telling him to come home. This entire scene was dropped in the syndicated version, and there was a jump from the end of the film to Woody standing at the door with his bags, saying his goodbyes.
A lot of risque jokes often get cut when an episode goes into syndication. This allows for more commercials and earlier showtimes.
One of the funniest moments from the Hank Scorpio episode of the Simpsons is cut from the reruns:
Homer is walking around the lair during an attack holding his coffee mug
Homer: Uh… you have any sugar around here?
Hank Scorpio: Sugar? Sure.
[takes two handfuls of sugar out of his pockets]
Hank Scorpio: There you go. Sorry it’s not in packages. Want some cream?
[reaches back into his pockets]
Homer: Uh… I… no.