It is easy. The L and R on the barrel are elevated and only on one side.
So, if you take them in your hand as if you want to insert them (flanges pointing inwards), this indication is on top if you have them the right way (left one in you left hand and the right one in your right hand). You can feel the letter with the nail of your thumb.
If you have them the wrong way around, the surface (on top) is flat.