Solar charging while in sleep mode

I may or may not have this same issue, I put my device on the window ledge in the sun all day, turned it on the next day and battery was the same.

Is there an easy/quick way to verify this works?

The device charges in sleep mode (well mine does anyway).  But the solar charging will stop when the device gets too hot to protect the battery, the issue is that leaving the device sitting in the sun without any cooling breeze from movement is likely to get it hot quite quickly.   So yes, you can charge the device when not in use by solar but it gets tricky in the height of summer.

I rested it on my windowsill and my house is 72°. I had it there for six hours and gained zero.

I've tried my 1040 solar like this multiple times, indoor,outdoor, low heat, breezy conditions and it never has charged while sleeping, let alone while powered off. 

Same here. 0% charged in sleep, or “on”, ever. In many controlled cases it was NOT due to temp issues.
Charging during activity seems to work nicely, but only then.