By the time I was old enough to recognize guest stars on Star Trek, it was already in syndication. Funny thing, I never really paid attention to the post-intro credits and the ending credits went by too fast. But recognizing stars of the day was fairly easy (costuming wasn't too elaborate to hide them).
Joan Collins was a movie star rather famous with adults in the 70's, but not to young kids. I didn't really begin to recognize her fame until I saw her popping up in various TV series during the 70's and 80's, only later on seeing her in blockbuster movies that came before Star Trek.
Frank Gorshin, Lee Meriwether and Julie Newmar were instantly recognizable from their roles on Batman. Yvonne Craig was more covert--her persona was so different from Batgirl. The Addam's Family was popular back then, so "Lurch" was easy to spot as Ruk. I wasn't really into the Western genre, so I missed recognizing a bunch of Star Trek guest stars that had done work in those kinds of shows.
When Battlestar Galactica appeared on the scene in the late 1970's, I instantly recognized John Colicos as Baltar (who had played the Klingon Commander Korr). And as time went on, it was fun to spot people who'd been on Star Trek that had roles in other productions. Certainly Teri Garr and Mariette Hartley were a few.