Some call the mohel based on the recommendation of friends. They have no questions. Their friend had a good experience and they want the same.
Some call many mohels, ask a few questions to each, and try to find the mohel that best fits their needs.
Parents who have been through the bris process before - whether once, twice, or more - more or less know from experience what they want the bris to be like. If they were satisfied in earlier brisses, they'll call their mohel back each time. (Unless they've moved states, or he's moved! Though I've had a number of people call me from NY, my old stomping grounds, and I still go back there on occasion for repeat customers)
In this past week alone, I've met with a couple of families dealing with their third boy (I did not do any of their previous brisses), and they articulated to me why they called me this time.
Their concerns included that a previous mohel did not and refused to wear gloves
In their most recent bris, the bandage came off and there was blood everywhere (here's why this should not happen)
They wanted to understand metzitzah and why previous mohels insisted on doing it the way which does not conform to their sensibilities (see also here and here)
ASKING IS ALWAYS THE BEST POLICY. Here are links from the Topical Index about important questions to raise before the bris
FAQ - Summary (links to answers to the questions) - Shorter Answers Appear in a Link at the top of the page
GOOD LUCK WITH EVERYTHING! Be in touch with your inquiries!