FiA F1 technical regulations 2014 wrote:19.7 Fuel approval :
19.7.1 Before any fuel may be used in an Event, two separate five litre samples, in suitable containers, must be submitted to the FIA for analysis and approval.
19.7.2 No fuel may be used in an Event without prior written approval of the FIA.
19.8 Sampling and testing at an Event:
19.8.1 All samples will be taken in accordance with the FIA Formula One fuel sampling procedure, a copy of which may be found in the Appendix to the Technical Regulations.
19.8.2 Fuel density will also be checked and must be within 0.25% of the figure noted during pre-approval analysis.
2014 F1 Technical Regulations 70/87 3 July 2013
© 2013 Fédération Internationale de l’Automobile
19.8.3 Fuel samples taken during an Event will be checked for conformity by using a gas chromatographic technique, which will compare the sample taken with an approved fuel. Samples which differ from the approved fuel in a manner consistent with evaporative loss, will be considered to conform. However, the FIA retains the right to subject the fuel sample to further testing at an FIA approved laboratory.
19.8.4 GC peak areas of the sample will be compared with those obtained from the reference fuel. Increases in any given peak area (relative to its adjacent peak areas) which are greater than 12%, or an absolute amount greater than 0.10% for compounds present at concentrations below 0.8%, will be deemed not to comply.
If a peak is detected in a fuel sample that was absent in the corresponding reference fuel, and its peak area represents more than 0.10% of the summed peak areas of the fuel, the fuel will be deemed not to comply.
If the deviations observed (above) by GC indicate that they are due to mixing with another Formula One fuel, which has been approved by the FIA for use by the team, the fuel sample will be deemed to comply, provided that the adulterant fuel is present at no more than 10% in the sample.
You are right. Ferrari could actually homologate a different fuel for every race. I doubt that it will be done by many teams, but it is possible. Nevertheless the FiA will always have the properties. It is the responsibility of the competitor that the properties are matched. If they cannot manage that due to evaporating losses they will be in trouble. I have no clue how the FiA will deal withg specific weight changes due to evaporation. Perhaps they will compare the values from the fuel sample with the values of the homologated fuel. A comparison should reveal quickly if a team artificially lets components of the fuel evaporate to generate heavier fuel than allowed. I do not even know if such a cheating strategy would be feasible. If it is I'm confident that something will be done to stop it.