You are all right and I have used all methods. With my old eyesight it is getting harder and harder to see the smooth side.
Jorge, I agree in essence with what you said, but I find that the results are not always reliable in the sense that it does not seem to agree with what I see as being smooth vs rough. Also, sometimes I see no curl or ambiguous curl in the sense that the paper gets an undulating curl making it hard to figure out.
I have tested Cranes, Strathmore, Bergger, and Lanaquarelle and this includes hot and cold press varieties and different weights from 90 # up to 300#. The cold press are obvious, but the hot press are more difficult. The textured ones are obvious, but the non-textured ones are sometimes ambiguous, more due to the low curl or the undulating curl when wet.
So, I agree with everyone, but merely add that in practice it is sometimes more difficult than not to make a sure determination. It may take some fine tuning as to how wet to make the paper, or how long to let it stay wet, or any number of things which is what is so much fun about the art of photography.
Some last thoughts.
Our local art stores have loose paper in large sheets. Sometimes they are stacked in any face order the clerk felt like, and it becomes impractical for me to wet test every sheet and I cannot tell with a good hot press paper which side is which, so the wet test becomes rather useless. Strathmore in pads is the best as the face side is always up in a pad, so I tear off a sheet and then notch the corner just as if it were film.
Sometimes, I have gotten cut sheets with the face sides of 2 sheets in contact, then the wire side in contact etc etc. This packaging is sometimes done by manufacturers who recut larger sheets and is done to prevent curl. It thus confuses the issue as you just about have to again determine the orientation of each sheet.