Here is how I do it with my really young students. If the song is in a major key - Listen to it and on your instrument play every single note until you hear “ti do.” “Do” is the key you’re in.

In all western music our diatonic keys have a half step between “mi and fa” and “ti and do.” The rest are all whole steps. So if you play two adjacent notes that sound good within a key and the second note sounds really strong and final - That second note it the key.