Judging is a most controversial area, here in the UK the National Blacksmiths Competition Committee hold regular seminars on judging for the shows to determine the UK national champions in both live forging competitions and static competitions for individual pieces. These seminars highlight the areas to be taken into consideration. Most who forge, know good ironwork and what it is composed of, and do not need to be advised of this, it is after all a personal judgement based on experience.
Basically be honest in your decision and be prepared to justify your decision, there are certain questions to ask yourself to reach that decision.
Briefly
Does it do the job it is supposed to do? That is is it Fit for purpose,
The number of skills used appropriate in producing the item.
The overall appearance and finish of the item
The suitability of the final treatment for where the item is to be used (indoor or outdoor)
The design, originality and artistic merits of the piece.
Make notes if necessary, if you are judging newcomers, be they young or old and wish to discuss any points, obviously point out the good bits, bits that could be improved and possibly why you chose the winning piece and what set it apart from the rest of the entries. Encouragement and praise for what they have presented also help to continue their interest in the craft.
Everyone has an opinion, and will let you know it, so, so long as you are fair to all, and able to justify your opinion if asked, all can learn and improve from the experience.
One incident that springs to mind was when a smith wasn't happy that he hadn't won the prize upon asking the judge why, the immediate response was "Its not good enough", this was then followed by a detailed account of what exactly was wrong (In the judges opinion) for the next ninety minutes. The smith got redder and redder, but listened, a few years down the line and he now regularly wins prizes and is an excellent smith. He obviosly listened and learnt from the experience.
Whatever you do Good Luck with it.