What's the best way to tell if a site was made with WordPress?

1) e-mail the owner
2) What Theme
3) WP Theme Detector

I prefer #2 over #3

Awesome, thanks for the response!

I just view the page source, and use ctrl-f to search through it for “wp-content”. If that exists you can be sure its WP, and if it doesn’t exist, and they are using WP, then it is so highly customized that it doesn’t matter. ??

If you want to search what a site is built with just try https://builtwith.com
That will tell if it is WordPress, and whatever else they might be running. They only index medium to large sites, and they are only right about 90% of the time, which to me is still good.

Another choice is the Wappalyzer browser extension.

https://wappalyzer.com/

Identifies not just the CMS but other technologies as well.