Find out the mains frequency.
The European grid ranges from Portugal over Estonia to Turkey. It is fed with alternating current, which has a frequency of approximately 50.0 Hz. The grid frequency (power frequency) is the same allover the grid, with the exception of local short-term swings.
At every moment the amount of electricity that is generated by the power plants must be equal to the amount von energy that is taken from the grid by the consumers. If the power demand increases beyond the power supplied by the generators, then the power deficit between supplied and removed power is taken from the rotational energy of the generators. They will thus slower, which means the grid frequency decreases.