Ask what they do about the moisture source, and listen to whether the answer is specific. Anyone who talks only about replacing wood is selling you a repeat visit. Ask for photos during the work, especially of what’s behind the siding once it’s open, because that’s your only chance to see it. Ask about the workmanship warranty length and what it actually covers. We ended up with http://www.igotrot.com/ partly because they document with before and after photos and carry a 24-month workmanship warranty. Also confirm the estimate is free and non-binding, most reputable outfits here do free inspections. Ask for a written scope rather than a one line total as well, it’s the document that protects you if something fails later.