The ideal "fair way" to run tryouts would be, to be 100% transparent. But to be that way, a lot of things from the players side would have to be transparent too. In the end, there is no such thing. Some tryouts are more fair than others, but none of them are totally fair. The coach has to look out for his team's interest. If he has a kid in mind, but the kid decides to go somewhere else, then he needs to have a plan B. But if tells the kid from plan B what's really going on, then the kid may not show up either. So, he has to play both sides sometimes. He also needs to compare his top picks to others, who are close enough, but not quite there. Just in case he needs to change his mind. So, he needs more kids at the tryout, than just the 20 that he picks. The ideal would be to have at least 50% more, so he can make comparisons. But he can't tell you "hey, your kid is just cannon fodder, I just need him on the ice as an extra".
This is why, a parent needs to do the leg-work before the tryout. Talk to the coaches. Ask the right questions, so that, if they are not 100% committed to you, you can still read them "between the lines". Do that with several teams, and then play the numbers game, to give yourself a good chance. We are talking about kids who are not "super-stars", of course, they are in a different category.
I was lucky in this regard, my kid never had to "really" try out, because I always got the "Yes" from the coaches before the tryouts happened. In fact, more often than not the coach called me before I called him. I guess, he is a good enough player to rise to the top 10% among his peers. Not quite a "stud", but not too far from them either. But I've been around all kinds of tryouts and combines (including the USHL combine), and talked to enough coaches, from AAA to A, to see how it works. If you go to a tryout without having a solid talk with the coach, then your kid better be one of the top ones. Otherwise, it's your fault if you get blind-sided.
I am also aware that if your son is a goalie, that's an entirely different universe. Almost like a "twilight zone". My comments are about skaters, not goalies.