A fifth might even sound off key if you're very used to equal temperament (it's about 2 cents below an equal temperament). You know it by there being no or less "wobbling" between the tones.
For listening tips, look for vocalist groups where there's "One Voice Per Part" (OVPP). Voces8, Vox Luminis, etc. When there's only one voice, you don't get the inherent wobbling happening when two instruments/voices play in unison.
Not all genres are possible to have just (jazz chord colors would sound rubbish).
You basically can look up just intonation versus equal temperament for the basics. https://pages.mtu.edu/~suits/scales.html gives the mathematical answer but doesn't get into the history.
A clause that says "assuming twelve-tone equal temperament" would be sufficient here, but you can really go down the rabbit hole if you start digging into scales (see microtonal), and your page is meant to be more basic.
