You should be well aware of the type of crowd you are dealing with, and the differences in crowd on a nightly basis. More generally, has there been an influx of a certain type of group – say drunken lads watching the football, or a stag party. A hen night can be equally as boisterous at times.
Be careful not to prejudice unfairly, however you need to be aware of how the crowd is behaving. Preventing any issues before they occur is always the best policy.
And this one is an extension of knowing the crowd. Not only do you need to know the type of crowd, you should be aware of how drunk they are too.
How long have certain parties been drinking? This involves knowing how long they have been in the bar, and at what stage they are at with their drunkenness.
A good door supervisor must police those that are drunk. By walking around and paying attention to those that may have drunk too much you are prepared should a situation occur.