How do I know if I am singing in tune?
You cannot tell reliably from inside your own head: you hear your own voice partly through the bones of your skull, which is why recordings sound wrong to you and normal to everyone else. The two honest checks are a recording of yourself against the original, and a pitch meter that reads the note you are actually producing and tells you how many cents sharp or flat it is. A hundred cents is a semitone; under about twenty-five cents will read as in tune to most listeners.