This writer has avoided judging photography contests.
If I were to do so, I'd follow the criteria below.
Go to What's a Good Photograph?.
A judge's lament - photo competition judging Bill Black
Catching the Judge's Eye Bill Hankins
How to Judge Amateur Photography Mark Goldstein
How to win photography competitions Haje Jan Kamps
Nothing Wrong with Photo Contests Mike Johnston
On being a Photo Contest Judge Dennis Rito
Photography Tip: How To Enter and Win Photo Contest Ken Deitcher