UV Panels ... how to find up side ?

It can be very difficult, if not impossible to tell which is the UV treated side of a polycarbonate panel. The UV side is indeed the "up" side, and when correctly installed polycarbonate will last for many, many years. If not, it will degrade in less than one year. So it is very important to get it right.

The only possible way is by the feel of the panels, but it's not necessarily the same from brand to brand. Generally though, the UV side will feel the slightest bit "smoother" than the untreated side. The difference can be, and sometimes is, imperceptible. If you know what specific brand of polycarbonate you have, you may be able to contact the manufacturer to see if they have any tips on being able to identify the UV side.

Other than that, install the panel and keep an eye on it. If you notice it discoloring (yellowing) in even the slightest way, remove the panel IMMEDIATELY and flip it over. Sorry I don't have any better advice for you, but good luck!