Symptoms
None, ever, until the moment it matters. There is nothing to hear, see, feel or test. The airbag warning light says nothing about this — the inflator is not faulty in a way the car can detect. The only signal available to anyone is the VIN.
Likely cause
NHTSA's own description: "the propellant in the inflators breaks down after long-term exposure to high temperature fluctuations and humidity. This breakdown can cause the propellant to burn too quickly, creating too much pressure for the inflator, and in extreme cases the inflator explodes, shooting shrapnel toward vehicle occupants." The Coordinated Remedy Order puts the consequence more bluntly: "that rupture causes the metal canister of the inflator to break away in hot, shrapnel-like fragments, which shoot out of the air bag into the passenger cabin and towards the driver or any occupants who are nearby." The propellant is phase-stabilised ammonium nitrate, and the recalled inflators are the ones built without a desiccant — a drying agent that would have protected it from moisture. That is why exposure over years, rather than distance driven, is what the recall documents talk about.
How should it be checked?
Check the VIN at nhtsa.gov/recalls before you go and look at the car, and check NHTSA's do-not-drive list at the same time. If the car is on that list, do not drive it and do not let anyone drive it to you — NHTSA's wording is "do not drive it until the repair is completed and the defective air bag is replaced", and manufacturers have towed and loaned in these cases. Parts are available. The repair is free and has no expiry. One caution about the lookup, and it is important here specifically. The tool shows only unrepaired recalls. A result of "0 unrepaired recalls" cannot tell you whether a Takata repair was ever done — only that none is outstanding. And NHTSA excludes "safety recalls that are more than 15 years old (except where a manufacturer offers more coverage)", which on Takata campaigns filed in 2015–2016 against cars from 2001–2006 is now biting. On a car that old, ask the dealer for the repair record rather than trusting a clean screen.
How can you check it yourself?
There is nothing to check on the car. Do the VIN lookup and read the do-not-drive list. If you can find out where the car has lived — the title history, the dealer stamps in the service book, the plate frames — that is worth knowing, because the campaigns are drawn on where it was registered rather than where it is parked.
The question to ask the shop
Have the driver and passenger airbag inflators both been replaced, and is there dated paperwork for each? Which states has this car been registered in? Has it ever been in a crash where an airbag deployed, and if so what was replaced?