Just Intonation intervals have "that sound", of calmly melting together and being all calm and pure and ancient sounding. For a Just 5:4 M3, you can start with a standard 12-tET M3 and lower the upper tone a tiny bit at a time; you will hear when it happens, you really can't miss it. The more complex the interval, the higher the overtones that melt together, so it gets more difficult, but with time you learn their characteristic sound. Northing corresponds exactly with the numbers, though- it is proabably easier for most people to tune the 7:6 minor third by ear than the 6:5 "pure" minor third, because 7:6 is low and dark like we usually associate with "minor" (it's like a 12-tET standard minor third wearing black leather and spikes), and 6:5 is wider and more bland than we are used to. Working with drones is a great idea when you are working on tuning stuff.