There are checks done to two of the cars after every race that goes beyond the standard procedure where they are dismantled and gone over in more detail and suspicion about certain parts will be a factor in deciding which cars are chosen and which areas to look at more closely. I'm also pretty sure that they have an identification system of the parts that goes beyond a simple visual inspection to see if it's a different design than it was before.Well the FIA clearly aren't aware of everything otherwise cheating would never happen on cars. If RB turned up one weekend with a new lighter chassis, from the outside how are the FIA supposed to know without it being declared? It's not like you can see the difference.
Whether thats actually possible/plausible I don't really know, but the improvement has been quite coincidental since that point.
I do wonder though how they managed to create this with the financial restrictions, and how Aston Martin basically made 2 different cars. No surprises they're the 2 teams under suspicion.
Otherwise there would be no point in having rules about a limited amount of engines, turbos, gearboxes etc since they would just as likely be able to hide that as they could hide a new chassis.
As for RB, if they used a new chassis without declaring it to the FIA when they already had the best car on the grid and both titles pretty much sown up already then they would be among the dumbest cheaters in F1 history.