I often get asked by my students, "Which side is the right side and which is the wrong side of the watercolor paper?"
If you are very particular and only want to paint on the top side of the paper, here's a tip for you. Do you purchase your paper in full sheets, then trim them down to quarter sheets to paint?
1. Look for the watermark
2. Using a pencil, lightly mark each corner with a small T.
3. Trim down your paper to quarter sheets.
4. You will now know which side is the top.
Having said all of that, if you're just starting out in watercolor buy high quality 140 lb. 100% cotton rag acid free watercolor paper and use both sides to practice on. The quality of the paper is the most important thing.
Many different brands of watercolor paper have a slightly different finish on both sides. Some manufacturers purposely make one side rougher than the other. Most of the time you can tell the difference between the top and the bottom, because the pulp is spread out onto a screen to dry. But there really isn't a right side and a wrong side to the paper.
Which side do you prefer to paint on?