Really though, it is all about the particular song/band you're mixing. Sometimes you might want piano in full stereo, or maybe mono panned to one channel. Same with synth stuff, sometimes both channels big full sound, or panned to one channel. It just depends on how much room you want to make in the mix with panning and such.

OK......what i'm asking is how to determin if the sound is mono or stereo. The end result of what i will do with it is another story.

errr
?
if the question is
... I have a stereo synth output and I want to check if it is a mono sound ?
... without listening ?... is this a trick question ?

HOWEVER
take one side of the output and reverse the polarity/phase.
If the sound totally disappears when mixed evenly together then I guess it is safe to say it is a MONO signal.

make sense ?

if it sounds like its coming from a huge speaker in front of you its mono.

otherwise, if its like in more than one place, its stereo.

its not that hard to tell.

my opinion is that if you don´t know how the sound was constructed you should always work your keyboards in a stereo track, cause all the keyboards are usally recorded from it´s left and right outputs at the same time (with the exeptions like when you´re taking the keyboard sound through stomp boxes cause then it definetly will be mono), and because most of the wel constructed sounds have layers panned a little left and right, mostly to add diferent harmonics or to thicken the sound......hope that is what you´re asking about