when you get an 80s film on dvd, how can you be sure you are getting the full unedited film as it was originally shown back then

Krull runs 121 minutes. The blu ray from Millcreek is the best release out there.
Genre films like Krull are being released now for the collector market, because it's mainly collectors now who buy hard copies. So the releases usually get the star treatment. Uncut , restored and looking better than ever.

Simply do a web search on the film of your choice. Details, like the running time of the film are provided. Also look for reviews by film critics. They typically note any changes to a film.

They were much more likely to have edited the film in the 80’s cinematic release than they were on subsequent DVD/Blu Ray releases.

In fact, later release usually include cut scenes (e.g., the Blade Runner directors cut).

I saw it opening weekend when it came out in the early 80's, I was 12 at the time, it was a fun movie the furst time I saw it, and it's fun when I watch it now, it's a film that never gets old. I downloaded the original theatrical version, as far as I know because it is a clean movie, it never needed editing for television, so you should have the full movie.