During a flute duet, a subtle beat can be heard when the musicians hold notes. are the two flutes in tune? how can you tell?

The flutes are not in tune. If the wavelengths are not the same, it will make, as the question tells, a subtle beat, therefore indicating that they are not in tune. If they were in perfect tune, you would not hear this beat, as they would have the same wavelengths.

Hope this helps!