The definition of equal temperament could not be simpler: the ratio between any two adjacent semitones is exactly the twelfth root of two. That is APPROXIMATELY 1.0595. The problem is of course in the implementation. Octaves are easy enough to do by ear, but for other intervals the ear will lead to one of the various natural temperaments, which will certainly give poor results when playing away from the home key. Electronic means would be possible these days, but I've never seen a piano tuner use an electronic tuner- just tuning forks and a lot of chords, scales, arpeggios and experience.