some flannels are double sided & both sides are pretty much the same & it really doesn't matter- or there is single sided flannel and you can really tell the difference- one side is soft/the other side is not- if you have a double sided flannel you can use which ever side you want.
if you feel you must know though- look at the salvage- the side that has manufacturer information on it will be the (right side) personally i have always felt that i pay for both sides of my fabric & use which ever side suits my need...i don't believe there is a (right & wrong) i've used many regular cotton prints back side when the more muted side fit in better than the brighter side....once the project is done no one knows that the other side is the side some would believe is the (right) side- all they see is the (correct) piece i used