Most of the time the best way to try out a player for your team is by having them play in a tryout scrim with your team. There's no better way to see how a player is going to perform and mesh with your team than by simply playing with them.
What you look for in a player is gonna be dependent on the goals of your team and the skill level you are aiming for, but generally indicators of a player you want to pick up are:
- players with good attitudes (don't tilt, good at taking/giving constructive criticism)
- people who are fun to play with
- players who want to improve
- players who's schedule lines up with your team's practice/match scheduling goals
I would take any and all of these traits over a very skilled individual player who lacks these qualities. It'll be a lot more enjoyable/educational of an experience picking up players who have good attitudes and have a drive to learn than players who are nuts but have poor attitudes or think they are above their team. Ovbiously skill should play a factor in your decision, but the things I listed above are things I look for no matter who I'm looking to recruit.
In the case of HL it can be difficult as a new leader to know exactly the best pickup for each class is, or even the difference between performances of certain classes, so especially when starting out it's better to look for good teammates over good players (though if you can find people who are both that is the optimal scenario).