How to evaluate a company if it is a Johnny-One-Note or not?

Q & AHow to evaluate a company if it is a Johnny-One-Note or not?
Fadh asked 5 years ago

Good afternoon Peter and KC,
 
I’ve been reading ‘The Intelligent Investor: Revised Version’ to find out more on how to evaluate a company after shortlisting and looking at its financial statement. In page 304 in the ‘Commentary on Chapter 11’ it stated that and I quote ‘The company is a Johnny-One-Note, relying on one customer (or a handful) for most of its revenue. …’. According to the book this is a NO NO. How do you actually find out if the company is as quoted? An example is appreciated.
 
P/S: Thanks for suggestion on reading the book. It is very insightful.