Umpires vary in attention span levels. Lack of concentration can be caused by a variety of reasons. An inflated idea of one's ability Lack of concentration can be indicated by inattention, slow reaction time, interest in things occurring off the field and the time gap between blowing the whistle and signalling. Inability to handle stress Indicated by over reaction to incidents. Can be cured by knowledge, experience and good coaching.
Body language is a good indicator.
Lack of confidence Indicated by hesitancy, slowness in decision making, over reaction and general demeanor on the field. Eye contact or rather lack of it between umpire and player can indicate unsurety. Often discussion with an umpire is the only way a coach will identify this problem. It may be caused by a number of factors, any or all of the above for example. Once again knowledge and experience will overcome this problem.