Art is subjective to an extent, which makes photography subjective… to an extent. There are a few things that if you see a photographer doing, you know they don’t know what they are doing.
How is the exposure? Does the picture look too bright? Or too dark? An excellent, or good photographer should have a firm grasp on exposure and be able to adjust his camera settings to look right in any setting. And if the lighting isn’t optimal in a particular setting, a great photographer will know how to work with flash, or additional lighting. If the pictures are too bright, or dark, you can’t use them. Make sure you when you’re checking out a prospective photographers work, you are keeping “exposure” in mind.
What is the composition of the picture? Are the subjects having fun? A poor photographer will never get people having fun in the pictures. It will seem like people are upset and perhaps the party wasn’t even all that fun. An average photographer, at the very least, will understand the basics of composition and where to be in order to get the best shots possible. A photographer who is great, or even excellent at what he, or she does will make your event look like it was the greatest event since the last Olympics. An excellent photographer understands composition well, and also how to position him, or herself in places throughout the night to get the best and most genuine photos.