Well I first sing it and then try to figure out the distance between the two notes. If I still have doubts, I use song reference like the “jaws” theme for a semitone and “so long, farewell” for a minor third interval. I hope this helps you, it sure helps me.
Minor Third- the first two intervals of the minor pentatonic scale, my mnemonic is Muddy Waters’ Mannish Boy “everything, everything, everything’s gonna be alright this morning! Semitone is quite discordant too, but because they are so close in pitch I can get this one.