And purple is the enemy
Only one person could properly get away with purple windows (RIP Prince!), but you'd never know it given the tremendous number of cars on the road with tint that has turned purple with age. It's an obvious sign that the film is of the most basic quality possible, a vinyl sheet with dyes that fade over time.
Look for tiny bubbles that distort your view
Make sure you check out how clear your view is. Obviously huge bubbles are a no-brainer red flag, but even very, very small bubbles cause distortion. A good tint installer will minimize the risk of bubbles, but if you've used a low-quality tint, they can form as early as six months down the road as the adhesive begins to break down prematurely.