AFAIK, QFF is derived from the calculated average temperature of the column of air between between the observation point and MSL. But you don't actually need to know that. Just compare the temp given in the question with ISA.
You'll prolly get a question along the lines of:
Pressure observations are measured at an airfield 1000' above MSL, where the temperature is recorded at +10C. Which of the following statements is correct?
a) QNH is greater than QFF
b) QFF is greater than QNH
c) QFF is the same as QNH
d) The question cannot be answered without knowing QFE
To answer this, calc ISA temp at airfield elevation (1000' therefore +13C), compare with obs (+10C). Temp therefore colder than ISA. Therefore, from aide memoire, Froid = QFF greater, therefore answer = (b)