This does amount to a certain degree of snooping, but its a way to see which WordPress plugins a particular website is using. Right Click on any empty space on the website as marked below. Unfortunately I could not get a clean screen shot.
OR one can simply use the Keyboard short cut key CTRL and U. The page source will open in a new window or tab. It looks something like this.
Use the Key Combo CTRL and F to open the search box and type in plugins. You would be looking for something like this.
Using this method, one can extract tons of information about the website like its theme, social links, etc. I cant really comment on the ethics, but I presume if the information is there to see, it can be seen.