when watching a movie or show on TV, I wonder if it's the full-length version or of how much am I missing. How's it determined?

If you are watching prime time on a major network. They tend to cut any nudity, gore, or major violence. Daytime TV has to be family viewing, evening is Teen and older viewing and after 10PM is a less restrictive.

Then they tend to put things on to end on the hour or half hour. Keep in mind commercials so a 2 hour TV show is just under 90 minutes movie and 30 minutes commercial. 2.5 hours is just under 2 hours movie. They just chop it. If the original movie length is 95 minutes after cutting the nudity and violence, then they cut another 5 minutes to be 90 minutes.

I don't know, I do know that I've seen movies on tv recently that I watched on tv long ago and I know they cut some of the movie to add more commercial time.

You could check somehow