do you play an instrument? I always try to imagine going key by key from one tone to another when in doubt—that helps a little, but isn't buleproof either. then I have this cheatsheet:

m3 - sounds unusual and the tones are not far appart
M3 - sounds OK, not nice, not special in any way, just OK
P4 - sound a little triumphant, but also a little just OK
P5 - sound triumphant like an 80's arcade game soundtrack, also I imagine a cowboy at a rodeo for some reason
M6 - sound special, but already passed the triumph
P8 - is when I don't hear the second tone at all, or I imagine the keyboard falling on ground accidentaly after pressing the first key

I didn't get further in tonegym, but tritone is quite easy as well, because it's dissonant, not the tritone itself but the note that pretents to be one in the 12ET system. m2 is also easy, it's like just someone doing a vibrato.

I play guitar. Trying to determine a flat fifth from min. 6th and Major 7th is frustrating as hell. Decent at min, maj. 3rds, p4 and 5ths. Those I've worked at a lot longer. Seems they can get confusing once I throw the others in the mix. Think it's just keep doing it thing.' Had to vent, lol.