In short, each car is assigned a scrutineer for the exact purpose of enforcing parc ferme conditions. Per the 2018 sporting regulations:
Article 34.7 states:
One scrutineer will be allocated to each car for the purpose of ensuring that no unauthorised
work is carried out whilst cars are being held under parc fermé conditions.
Any of the changes to the car permitted under parc ferme conditions must be made in their sight (Article 34.2.x):
Any parts which are removed from the car in order to carry out any work specifically
permitted above, or any parts removed to carry out essential safety checks, must
remain close to it and, at all times, be visible to the scrutineer assigned to the relevant
Furthermore, any parts removed from the car in order to carry out any such work must
be refitted before the car leaves the pit lane.
Certain data is also taken from the teams prior to qualifying (Article 34.1):
Every team must provide the FIA technical delegate with a suspension set-up sheet for both of
their cars before each of them leaves the pit lane for the first time during qualifying practice
Random checks are made post-qualifying to enforce both these and other rules (Article 34.3):
At the end of the qualifying practice at least six cars will be chosen at random to undergo further checks, once informed their car has been selected the team concerned must take the car to the parc fermé immediately.