WP XRAY from Sarah Staar is a very nifty tool and as the description says, you can spy on your competitors and see what they’re doing.
“INSTANTLY Reverse Engineer Any WordPress Website. Discover Which WordPress Plug-ins & Themes A Website Is Using
and a whole lot more…”
Not only can you see what themes are being used, you can also find out if the company has other websites.
The only downside I can see is it can only be used for sites using the WordPress platform.
Sarah gives a step by step explanation on exactly what you can do with this FREE plugin.