Get within six inches of the glass in direct sun. That's the only inspection that matters, and any shop confident in its work will hand you the flashlight.
Look at the edges first. A good cut follows the glass line with a consistent hairline gap — not a wandering one, not a gap you could park a business card in. On door glass, roll the window down two inches and check that the film is trimmed to the top edge cleanly.
Check the corners for lift. Check the dot matrix band at the top of a windshield or rear glass, where film has the hardest time adhering. Look for creases, which are permanent, and for fingers — long thin channels where the film didn't lay down.
Look for contamination: specks of dust or hair trapped under the film. A couple of tiny particles on a large pane is realistically the best any shop can do. A constellation of them means the glass wasn't prepped or the environment wasn't controlled.
Now the two things that are not defects. Haze, and small water pockets that look like bubbles, are normal for the first week or two while the mounting solution evaporates through the film. Don't scrub them, don't roll the windows down for a few days, and don't panic. Any shop that didn't warn you about this in advance is going to earn the phone call it gets.