It depends on how sophisticated your load balancer is. Some load balancers can verify only that each server is responding on the necessary TCP port. They'll generally connect to TCP port 3306 (or whichever port you're using) and assume the server is unhealthy if the connection is refused or if it has to wait too long for a response. Some load balancers are more flexible. They might give you the option of scripting a complicated health check or of running the health check against a different port than normal. This provides a lot of flexibility and control.
Re: Health check for MySQL
It depends on how sophisticated your load balancer is. Some load balancers can verify only that each server is responding on the necessary TCP port. They'll generally connect to TCP port 3306 (or whichever port you're using) and assume the server is unhealthy if the connection is refused or if it has to wait too long for a response. Some load balancers are more flexible. They might give you the option of scripting a complicated health check or of running the health check against a different port than normal. This provides a lot of flexibility and control.