I's say by comparing what you see on the ice at the beginning of the season and what you see at the end of the season. Did they improve or was it a steady decline of performance - were the right adjustments made to the line-up. Add to that how disciplined the kids are on and off of the ice (are stupid penalties a problem throughout the season? Does the coach bench chronic behavioral problem kids?) and how the coach manages poor attitudes...also, does the coach have a consistent approach to the kids or does the coach play favorites.
That was my initial thought as well - are the kids developing and playing their best hockey at the end of the year? Do the kids respect him, do they respect his decisions? How does he "teach" - is he a screamer or does he speak quietly to the kid and let him know what his expectations are?