Which jockey wheel on top? I've just bought a new pulley set for my rear derailleur but do not know which one goes on the top (nearest derailleur) and which one goes on the bottom. The
wheels have arrows on them;vI suppose they indicate the rotation direction for the pulley wheels when rotating 'normally'.

PS : I see that one pulley wheel is beefier, thicker than the other one 
Derailleur is a M970 XTR
Here's a photo of RD-M970 from behind. As you can see, the slotted (guide) pulley goes to knuckle and solid one (tension) to the bottom of cage. I'd align the arrows with chain movement direction that happens when you pedal forward.