[amagras]

Go back to the source, for example, singers have one mouth = mono, bass and guitars when recorded with one mic or DI = mono, keyboards that have 2 outputs = stereo. Insert a correlation meter on the Master and check out which of those tracks actually have stereo information, if they do (could be reverb for example and the meter will show information in the sides) leave it stereo, if they don't (the meter shows a vertical line only) insert a la PT stock plugin called Downmixer to convert them to mono or simply split into two mono tracks and delete one... That's how I do it here.