There is lots of talk here on what makes a photographer professional but what about models?

Professional models are the models who make more money than women in the Oldest Profession.

;-)

They show up on time.

haha.. and they know exactly what they're doing

Like everyone said they are on time and ready to work. They know exactly what they are doing when it comes to posing themselves. They do not need much guidance. They act like professionals at their craft, not like lost children needing to be told how to do things.

To me, a professional model does the following:

Shows up on time or early to shoots ready to work
Is paid
Is organized
Needs very little to no guidance when it comes to posing
Knows how to tell legit from unlegit
Is either promoted by a legit modeling agency or a very respected freelance model.
Understands model releases and knows how to communicate with a photographer and knows how they work.
Knows when a photographer is going too far and leaves when pressured into doing something they are not comfortable with.
Doesn't end up doing what the unlegit photographer wants and doesn't go to a message board whining about it.