At my Catholic college we are blessed with many priests nearby. There is the chaplain, Father B.J. (not his real initials), the young friar, Father Firstname, the old monk Father “Call me by the first name because the last name’s too hard to say”, and the beloved and also fairly old monk who is referred to by pretty much every student as Father Lastname, but whom I have once or twice heard introduce himself as Father Firstname. Most people wouldn’t dare call him by his first name I think, at least not students. I smiled at the sign for our Penance Service with Individual Confessions–we had the two Father Lastnames listed that way, no first names, and then the others were Father B.J. Lastname and Father Firstname Lastname. I like whoever made the sign.
But basically, I usually call a priest what everyone else calls him, or if in doubt, just “Father.”