Fuel tanks must be made of materials approved by the FIA and have single rubber balder manufactured by approved companies.
The tank must be placed directly behind the driver and directly in front of the engine. All fuel lines lines must be self-sealing in the event of an accident and no lines should pass through the cockpit.
Fuel tank must be covered within a chrusable structure that forms part of the safety cell. The structure must be able to withstand very high impact loads.
FIA at any time grand prix can be from any of the car to take a liter of fuel as a sample to check its legality.
FIA F1 2017. technical regulations can be downloaded here.