Well that's the theory anyway. In fact they look under the hood and if it looks generally "stock" they don't look further. If a complaint were to be lodged by another competitor then they might go so far as checking part numbers or casting numbers, but they frankly don't as a rule.
People are spending upwards of $100K on mods to the faster cars - and that's not including the price of the car. The lengths some of these guys will go to reduce weight is unbelievable. Hollowed out batteries with an AGM inside, passenger seats made entirely of styrofoam that would never support any weight, window regulators removed except the driver's door and replaced with aluminum supports for the glass - I could go on and on. Exhaust manifolds duplicated from cast aluminum, then spray coated with cast iron so a magnet will stick.
"pure stock" - I'd love to watch the tear down.
The cars are part numbers checked during registration. All the cheating is on the inside.