Tryouts provide an opportunity for both coach and player to get to know each other. I believe the head and assistant coaches should conduct the tryout session(s) rather than stand on the sidelines.
In my opinion a tryout should be the same as any other normal practice or formal training session. You need a warm-up, technical footwork, small group activity or 1v1 game, and an expanded game in the end (5v5 or more).
Schedule 2-3 hours depending on the age group, this includes 30 minutes prior and after the training session, for example: