you will never really know if they are a flake, but some of the warning signs are they respond back with a single sentence or a few words when you ask them questions in an email. Also them not wanting to provide a phone number so you can talk in person in advance. The new girls or really young ones are the worst, they either get cold feet or stay out partying the night before.
If someone has been on MM for a more than 6 months but only 4-5 photos in their portfolio, they are likely not all the serious.
Best advice talk to them on the phone in advance and you will get a good feeling as to how serious they are. Express to them you are busy, and that you have a number of girls who want the session time, so you only want to deal with someone who is professional and will be on time. I have even jokingly asked things like you don't plan on being sick that day, or have a seriously ill relative that could prevent you from attending?
hope this helps