You’ve seen the videos of what a really well cut diamond can look like so you might just have to use your eyes to select from the options that are available to you. You’ve done a good job of narrowing your choices to a selection that should be better cut than most diamonds that are out there. Now you want to look for a nice edge to edge sparkle, a good level of brightness, and a pleasing symmetric facet pattern across the whole face of the stone. The stone should have lots of sparkle moving across the whole surface, and it should have patterns of bright and dark facets, turning off and on across the whole surface of the stone. There should be no areas that remain dark across different lighting sources or as you move your hand. The whole face of the stone should look active with light play. Be sure that you look at it in natural daylight or incandescent light because high spotlighting like they usually have in jewellers makes it hard to assess cut.
I also recommend taking a picture with your phone at the jewelers. When I was first looking for a diamond, I saw a stone in a store that was so sparkly and bright in their lighting. I took a quick pic with my phone and the jeweler said "oh diamonds don't photograph well with phones." When I looked at the picture later it was awful, mushy and dull. The next day I found PS and JPs videos - I learned that it was a terrible cut and leaking a ton of light. I almost made a very expensive mistake and that picture saved me.