You know, if you feel uneasy about it, just call them Dr. _____. If they REALLY want you to call them by their first name, they will tell you that in person.
I call many of my professors by their first names because that is how they communicate with me (sign emails, answer phones, etc). As my general rule, I always start out with Dr. ______ during every new communication. (this might be strange, but it's worked for me).
The PI I want to work with, for example: I contacted him by email and called him Dr. He responded and signed with his first name, so I responded with his first name. But, when I talked to him on the phone the first time, I called him Dr. again. Eventually it will move to the first name too.