How can you tell if a panel in a string system isn't working?

How can you tell if a panel in a string system isn't working?

Since the panels are all connected to the same inverter, any issue that causes one panel to produce less electricity (such as shading) will reduce electricity for all other panels on that string.  It is therefore difficult to detect a malfunctioning panel.
If you are generating less than what was predicted this is an indicator and your inverter monitoring system may be able to assist you by describing any faults or errors. Almost all inverters have red and green indicator lights indicating whether they are functioning well, which you can check on a typical sunny day as a good starting point.
It is also good to check what has changed near your solar system if there are any new objects which cause overshadowing on a particular panel. 
Alternatively have a qualified solar electrician check out the panels.

SEE ARTICLE 'How can I tell if my solar system is damaged or underperforming?' FOR TIPS ON ASSESSING SYSTEM PERFORMANCE