2. Check out their references
3. Check with the Better Business Bureau for any bad reports
4. Check on recent jobs and try to find out how many times the framing inspection was turned down by the Building Inspections Department before it passed.
5. Look at work that has been done over one year ago and see how the structure is holding up. Look for cracks in the drywall, doors that do not close properly, etc. that could be a sign of shifting. (There are other possible causes for shifting such as poor foundation or loose sub grade. These could also show up as cracks in the foundation walls and have nothing to do with the framing.)
When the work is being done look for missing nails or fasteners where there should be some, loose walls or sill plates, tight fit joints around door and window headers, nails in plywood sheathing that are not hitting the rafters or joists, etc.