to find the tempo of a song you didn’t make, you can try remaking a maybe 10-second portion of the song then playing that portion of the song and what you made simultaneously. then if the remake starts slowing behind then you need to make it faster, and if it is ahead you need to make it slower. for example, by doing this i found out that the tempo for megalovania is 120 BPM. if you want to be more accurate then you will need to use a longer portion of the song.
or you can just like find an accurate midi of it online and look at the data or something.