What do you do to prep/clean a vehicle with ceramic coating on it to apply new decals? Or is there nothing you can do with a ceramic coated vehicle?
It depends... And there's several different levels of products.
Professionally applied ceramic coatings are the worst. They need to be wet sanded and buffed to remove, they can last 3-5 years before they start breaking down enough to remove easier (without sanding or buffing), so the best of the best products screw you for laying graphics without a LOT of prep work. Dealerships don't usually have cars professionally done unless they're high end, but a lot of people will buy a new car, take it to a pro detail shop for ceramic, then decide they want graphics or a wrap.
Then there's the DIY grade ceramic coatings. Those you can usually get off with chemicals, sometimes alcohol will do it, sometimes you need something stronger. With the better ones you might have to buff as well, but it takes several cleanings, and it doesn't hurt to clay-bar as well. They break down in a year or so, that's when they're easiest to remove. This is usually what dealerships and individuals use because they're economical, and they can apply them with no special equipment or skill. Dealerships will often not disclose either, salespeople often don't know what their prep people did to a car, especially used cars.
Easiest is the spray wax type ceramic, going over those 2-3 times with 99% alcohol usually gets them off. They start breaking down in months, much like good ol' car waxes, they just contain some ceramic in the formula.
How to tell is by look, and how water runs off, it's different than waxes or just fresh paint, you should be able to tell. The appearance will look almost like a fake gloss, towels will wipe across with no friction... When the water no longer runs off, you usually have it removed. Best practice is inquire up front if they've had ceramic, and if they have, or you determine it's been used, give no warranties. Ceramic is formulated for nothing to stick to it, not dirt, not liquids, not vinyl....