I don't know how Karl sees leakage in that awful video but take him at his word.
You ask a good question, what does a good one look like?
It took me forever to even get the most rudimentary understanding of this, but watch as the video clicks around. The facets should turn on and off, not stay static or the same for several turns. If several stay dark, they'll stay dark on your hand and you won't be thrilled.
Keep trying. It's worth it.
I have the same problem as you. I can't tell by watching a video. I ended up buying a Jann Paul EC. I haven't received it yet, but I've seen it in multiple lighting conditions and the sales person had eyes on it. Also, JP has a reputation for their quality control. I would say to go with a company that has a good reputation for reps that give honest opinions. Also, make sure you know their return policy. Narrow down your choices and order. Get yourself an ASET scope (about $50) and a diamond ring holder ($10 on Amazon). When you get the diamond, wear it into all types of lighting and look for those flashes. Remember, it is a different type of flash than other cuts. I don't have mine yet or I'd post lots of videos!
Also, here are some videos to look at:
Lauren B - EC- Clarity
Good Old Gold
Lauren B-2