(Note: answer edited after Matt L.'s comment below; the definition of fundamental frequency given here was wrong).
For a sum of sinusoids, the fundamental frequency is the greatest common divisor of all the involved frequencies, assuming it exists. In your example, the fundamental is 50 Hz.
When you're dealing with a signal that is the result of adding several periodic signal together, be careful to verify that the result is periodic too. In theory, if you add two period signals and the ratio of their periods is not rational, then the result is not periodic.
I've never heard about the "frequency histogram method". You can, however, calculate the signal's Fourier series and just pick the harmonic closest to zero as your fundamental.