Ask for and check references! Anyone can come up with a list of customers. When calling, ask about quality, turn-around time, etc.
Ask the contractor to provide a list of work that has been completed nearby. Drive by a couple jobs to look at the quality of the job, the neatness, consistency.
Things to look for: Are there streak marks? FYI: These are avoidable! Only a small percentage of driveways are susceptible to 'streaking' - the ugly remnants of bad squeegeeing technique. How do the cracks look? Messy crack filling is a sign of weakness.
Look for trustworthy individuals. If the sales representative is pushy or sketchy, you might want to look elsewhere! Chances are the quality will be similar.
Professionalism. Are you dealing with a legitimate company? A quick Google search is the best way to dig up complaints.