I suppose make sure the server has a ball in posession and is ready to serve. A few situations where my team was beckoned to serve and server didn't even had a ball nor was even close to end line.
Other situations where receiving team didn't even had six players on court, when a player went to retrieve a ball and now is tying shoe lace off the court.
- I got an out of rotation call on a few of those situations.
I have to laugh at the few occassions where wrong team was beckoned to serve.
Pay attention ref!