3.) Check That They are Licensed and Insured

Are they fully licensed?

Most states require that contractors be licensed to do home improvement work. The requirements to get licensed may include a written exam, proof of financial ability, letters of reference and experience in the field, depending on your state.
Do they carry liability insurance?

Liability insurance helps to protect your property in the event that your contractor causes any damage while on the job. Without liability insurance, you could be left to fix any damages if something goes wrong.
Do they carry workman's compensation insurance?

If a worker was to get hurt on your job, you are protected if the contractor has workman’s compensation insurance. It’s important to not underestimate the importance of workman’s compensation; if a worker gets hurt on your job site, you can be financially and legally liable for their injuries, and most homeowner policies do not cover these types of accidents.