Two quick answers:
- 1. The rougher side. The “felt” side (the side you paint on) has more “tooth” than the “mould” side.
- 2. The side where the watermark reads correctly. If you hold the paper up to the light, you should be able to read the watermark properly (ie it is not reversed “mirror-writing”).
Note that this is not vitally important, you can use both sides.
“At St Cuthberts Mill, we endeavour to make all our paper so that both sides of the sheet can be used, so there’s no definite right and wrong side. However, most artists prefer the Felt Side. The Felt Side will have a more random texture to it’s surface, as it doesn’t have the regular pattern of the wire mesh of the cylinder mould it was made upon. The Felt Side is also more rigorously tested during manufacturer.”
I use Saunders Bockingford paper from St Cuthberts Mill. The felt side is on the outside of their rolls, and I’ve never found the watermark – the company says they make the watermark almost indistinguishable to facilitate using both sides of the paper.