During a new pitcher's warmup pitches....

[waltjp]

I think the thing that would first tip anyone off about a rookie blue is timing. Many new umps time to time every call to coincide with the play and this will eventually lead to trouble. Have someone video tape one of your games. It doesn't have to be the entire game, just a couple of innings will do. Make sure to keep the tape running between innings too. You'd be surprised at what you see.