I'm covering the chorus of a song in a music composing program. What do you do when you're trying to do that and you have no clue what the notes are?

Guess and check, really.
If you have access to a tuner or a spectrogram, you could play the song through that. That's what I'll do to cover a song If you have an Android, try the free PitchLab. It has a spectro mode that displays keys

(maybe you should post this in the music production thread, but dunno)