Here's my short answer since I'm trying to get out to Mexico for our youth ministry Spring Break trip...

There are many indicators one could use: students following Christ, number-indicators, the implementation of all 5 biblical purposes, etc... But if I had to narrow it to one indicator of health I would say "volunteers owning ministry."

The reason that I quantify volunteers is because anyone can volunteer, but volunteers who own ministry are a notch above volunteers who feel a sense of duty ("I do this because no one else will"). When you feel ownership of ministry, you care about students and are willing to sacrifice and "do what it takes" to make a youth ministry strong. Leaders who feel ownership of the ministry make up the foundation of a healthy youth ministry. A youth ministry with quality volunteers is always within reach of health, but a volunteer without quality volunteers can't be fully healthy.

I believe volunteers are the ones who should be doing the youth ministry, not waiting for the "paid" professional to do everything. Typically, the lead youth worker doesn't last very long and it's the volunteers who keep the ministry running. Bottom line: when volunteers feel ownership they care more.

http://www.simplyyouthministry.com/community-articles-doug-s-thoughts.html