How to know what they're looking for at audition
Honestly, it's almost impossible to know what exactly a casting director is looking for at an audition. Most of the time they already have an idea or imagine of what they want in their mind, and when that walks into the audition, they know they have found what they are looking for.
Casting breakdowns are there for a reason though, so if your going for a character who is described as a 'hunk', don't go in and play it as the opposite. Read the breakdown properly as it actually tells you what they are looking for. When it comes to the specifics of the character that your auditioning for, if it isn't in the text then you simply have to make your own decision on what you think the characters history/motivations are. If you have thought through it properly and come up with a solid character which is distinct then you are going the right way. Most people will go in without a proper idea of who or what their character is, and so it comes across as very general. You need to make decisions and be specific.
If you have got the script on arrival, and only get a few minutes to read through it before you go in, then make quick decisions about the characters and if you are unsure about something, dont just skim over it, ask- they want someone who is serious, not someone who cuts corners.
Apart from that, make sure you are early (not too early though as to cause an inconvenience), make sure you listen to what is said to you, and be polite (not just to the CD, but to everyone that you see when you enter the building).
So as I originally said, it is impossible to know exactly what 'they' want at an audition, but if you read the casting breakdown properly, create a solid character and are professional, polite, and on time, you will only be helping yourself.