Let me start by vaguely explaining the process. The prep work (cleaning the outside for cutting and shrinking) comes first. Then the cutting of the individual windows and shrinking the film comes next. After that the final step (the most critical), is the installation.
After you understand the process you can better understand the importance of the installers job. So one question you should ask is what type of experience does the installer have. Also where they will be tinting your vehicle. Make sure they are not just tinting it in some dirty garage or outside. You want the environment to be controlled with the least amount of dust or dirt.
The brand of the film and the warranty are very key. You don't want to pay for a film that will peel, crack, bubble or fade on your vehicle in the coming months or years. You want a film that has a warranty against all these things. Most professional tinters will carry Lifetime Warranty Films. In my professional opinion Llumar window film is the best available because of its durability, scratch resistant coating and over quality. This is coming from years of working with all different types of films.
You should always ask about the state laws and make sure you have a good understanding. This will help you make a decision on what percentage tint you put on your car. Keep in mind that people get their windows tinted for numerous reasons. Hopefully the person you talk to is very helpful with getting the right percentage for you and your family! A lot of the time they may just help you with this when you come in to get job done.
Other questions you should consider asking, how long it will take and can you wait? Be very skeptical of them getting your vehicle done fast. They are more than likely trying to get quantity rather than quality. Can you wait and if you can how comfortable will you be? The average tint job may take up to a couple hours. So if your sitting in an uncomfortable waiting area with nothing to do for hours, you may want to consider your options.
So in review:
What type of experience does the installer have? Where are they tinting your vehicle?
What brand of film are they using and what are the warranty options?
What the tint laws for your plates are? What percentage is right for you?
How long will the job take? Can you wait if you need to and what do they have to offer while you wait?