Used solar panel testing cloudy day

• measure the open circuit voltage (Voc) in any direct sunlight. Compare all panels (same brand/model) together at same time/temperature/angle to sun--The ones that are "different" (usually lower voltage by 10% or more, are questionable). Voc is temperature sensitive and hot panels may have 10-20% drop from Voc-std on the back of panel label--That is OK and normal.
  
• measure the short circuit current (Isc) in full sunlight with panel pointing at sun. First (panel away from sun, or something dark over panel, connect the + and - panel leads together (short). Point at sun, use clamp meter to measure current. Note that Isc is proportional to amount of sun hitting the panel... Nominally if measured current is >0.5*Isc in full mid-day sun--probably OK (since you have multiple panels, quickly measure in panel the same way, quickly between all panels... If there is (one or more) different by more than ~!0%, probably something is wrong).
• Look at panels closely... You are looking for brown/black areas under panels, looking for water or corrosion, damaged face glass, damaged rear plastic (note that if panels are dirty--Brown spots on surface of glass from leaves/dirt can look like bad cells/wiring underneath--particularly where water pools near the frame).
• Look at all panels and find any "differences" in coloring/look of solar cells. Not all differences are "failures" or potential failures (some cells can reflect light differently as they age)--But it is usually not a good sign. Cracked cells are bad (there are some strange lines across cells that can look like cracks, but are not--Unknown if this is or will be a bad panel)
  
• Look inside j-box for corrosion/burned terminals (if that type of J-box). Solar panels are not really repairable (shorted by-pass diodes in junction boxes you can open can be replaced).
• Look through glass above J-Box--Sometimes an overheating J-Box can brown (light coffee color, or "rusty" haze) the area under the surface glass.
  

-Bill