The easiest place I've found to test mine is at the panel itself where the lead wires are attached. Others may have a different suggestion.
Prevailing wisdom has it that monocrystaline panels degrade at about 0.5% per year. The Real Lifespan of Solar Panels - Energy Informative
We still have the original 10W panel that our Journey came with. (rectifying that soon
) Based on the above, one would expect about a 5% or 6% degradation since 2007. I measure closer to 10%. That seems to be consistent the experiences of neighbors and fellow travelers we've spoken with. But, technology keeps improving!
Fair Winds and Following Seas