Well, I got a photo of the bad spot, on a forum here:
http://www.wind-sun.com/smf/index.php?t ... 8#msg13858
(dang - must be member to view photos there, sorry)
I'm pretty sure the easiest way measure it, is to switch off the inverter,
have the fellow on the roof disconnect one string, and reconnect inverter.
Use the inverter to measure volts/amps. Disconnect, switch strings, and remeasure, disconnect inverter, connect all strings and continue to run system, and hope enough of a loss shows to see the 2 strings are not balanced anymore. If not now, I'm sure the "spot" will keep growing till it has an effect. I guess I have 20 years to make the claim.
I also found there is a lot of RTV over the panels, and that's a PITA to clean all the bits of it off.