You should have been or will be supplied with the criteria for judging. If not, is there a fair booklet describing the entry qualifications? You can then judge initially, at least, on how well an entry meets qualification.
I have NOT judged but have acted as assistant. The judges had a rating list and determined how well the entry did with each item in the rating list. Individual entries were scored and places awarded or in case of ties, advanced standards were applied.
I would do a quick web search, were I in your position to see what forms may be available.