Company will come out on a service call and try to sell you the most expensive equipment trying to hit a home run. Make sure you do not buy on the spot and get other estimates. Any reliable company would be able to install it within a week.
No Better Business accreditation or a very low grade. There are many companies locally with A or A plus standards. I would not use anyone that is lower than this. Many times the lack of accreditation is worse than having an F grade.
Minimal longetivity in the workplace. Some companies will come from different cities for the summer or the winter months and then when you need them you have to call their home office in nowhereville.
Cold Calling. Any company that has to cold call for business or telemarketing usually has something up their sleeve besides a low initial offer to set their foot in the door.
Limited company information. They have no web site or have no affiliation with a manufacturer. Also some companies that take the manufacturer sticker off and put their own (private branding) will be very, medicine very expensive.