Find the Bass
Lukitoh...Have you got a "favourite" song, one that plays over & over in your head. Get comfortable with the key, with the pitch. From there, it's all just math. Compare the bass note of the new song with that of your most favourite. I'm probably not explaining this as well as I would like to.
As cassp stated...perfect pitch is a definite plus, but we don't all possess that luxury.
Good Luck in your endeavours


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...L