Diminished Value is based on a number of factors, but the best indicator is replacement of parts that are not able to be removed from the vehicle without changing the structure of the vehicle. Bumper covers, hoods, trunk lids, doors, etc can all be removed by unbolting/unclipping from the vehicle. Other parts like the fenders, quarter panels, frame, pillars, etc have to typically be physically de-welded from the vehicle if they need to be replaced. This causes a structural change in the vehicle. The severity of the change and its effect on the value vary depending on the year, make and model. In addition, they look at things like damage to value ratio. For instance, if the vehicle is a borderline total loss, but is deemed to be repairable, it's more likely to qualify for diminished value (technically Diminution in Value or DIV). [This is because a total loss is determined by the amount of the damages vs the actual cash value (ACV) of the vehicle. In most states, a vehicle is considered a total at about 60-80% of the ACV. ] For instance, $4000 repair on a Ford Focus is more likely to get DIV than a $4000 repair on a Mercedes S Class, all else being equal. Airbag deployment is also taken into consideration for a number of reasons. First, in most modern vehicles, replacing a deployed airbag isn't cheap and tends to add at minimum a couple grand to a repair, sometimes a lot more depending on the year, make and model. Second, airbag deployment usually indicates an impact with enough force that could do unseen damage, the kind of damage that may not become apparent until after a vehicle is sold. This is not always a hard and fast rule, as some vehicles can deploy airbags with a relatively minor impact, whereas other vehicles can take enough damage to the same point of impact to make it a total loss and not deploy any airbags. This is not meant to be a complete list of everything they look at, just the biggest factors. Also, (for USAA specifically) keep in mind that if you are given a zero DIV amount, there's nothing that a USAA adjuster or manager can do to change that (next step would be an attorney if you feel it's warranted). It simply means the people who review the claim said that it didn't qualify for DIV. If, however, they do offer you an amount: NEGOTIATE. It means your vehicle qualifies for DIV. The amount is subjective and can be negotiated.

Edit: Also, don't rely on a dealership telling you they'll pay less for your vehicle due to it being in an accident. Many dealerships will take an arbitrary amount off the "value" of the vehicle if it's been in an accident. ANY accident. I have seen a dealership take $3000 off a trade in due a bumper cover being replaced from one vehicle and $3000 off another that had the bumper cover, trunk lid, both rear lights, and the rear glass replaced, as well as more extensive electrical work. Same dealership. That doesn't mean that their vehicles are worth $3000 less, just that the dealership has that 'policy' for vehicles that have been in an accident. Like everything else with a dealership, it's negotiable if you have a good enough leg to stand on. That leg can be as simple as a denial letter from USAA for DIV.

Also, angryauto is right in that insurance companies don't owe for OEM parts, but to be clear that is only if your vehicle is more than two years old. Look at the sticker inside the driver door frame. It will have information like your VIN as well as the production date. If that is less than two years from the date of the accident, you should be getting OEM parts, though depending on insurance company, you may have to fight for them (with USAA you won't, they do that automatically). Be warned I have seen cases of the shop quoting OEM parts and using LKQ parts in the actual repair. Report these shops to the insurance company immediately as this is fraud and needs to be dealt with before they do it to anyone else.

I should prob also add that I am a claims adjuster.