1. Remember, money is a skill. But not always! Sometimes someone who is a natural at headshot photography and is just getting started in the business can give you great headshots for a low price! But always look at their sample headshots first, because you don't want to spend even $20 if the headshots you get back are absolutely useless.
2. After seeing a sample of headshot photographer, give them a call assuming they are in your headshot photography price range. You can email them but it's a much better idea to call because you want to get an idea of what kind of person they are by talking to them over the phone. Do they have time for you? Do they care? Do not work with them if they do not seem to be interested in talking to you, or seems busy to deal with you.
3. Email to set the time and make sure they are responsive. This applies to every professional you may be working with: if they take more than two days to respond, it's not a good sign. You should always feel comfortable knowing that your headshot photographer is prepared and ready to serve you. You pay them, so why should you worry that they might not show up?
4. Once you feel comfortable that your headshot photographer is responsible and easy to work with, it's a good idea to see if you can meet with him prior to the session as well. But many headshot photographers are very busy and can not offer their time like that, so if that is not an option, you do not need to worry too much. Headshot photographer should be able to make you laugh and you have to feel comfortable around him/her, because this is the only way you'll get a good picture.