[throwaway13qf85]

It's tough for someone who's not a professional musician to tell the difference between a just fifth (3:2) and an equal tempered fifth (2^(7/12):1) but it's not impossible.

Probably the easiest way is to go to a piano and play a fifth with the base note at middle C. The sound noticeably changes after a couple of seconds. That's because the ratio isn't exactly 3:2 and the frequencies of the pure notes start to get out of alignment (the same phenomenon causes 'beats' when two similar but not exactly equal frequencies are played together).

It's even more pronounced when you play a C major chord (C-E-G) because the major third (C-E) isn't exactly 5:4 and the minor third (E-G) isn't exactly 6:5 either.

[theOnliest]

The equal-tempered major third is 14 cents higher (1 semitone = 100 cents) than a just-tempered third, which is pretty noticeable even for non-trained musician.