Most of the time the website using WordPress are having a particular pattern in their code section.
If someone wants to whether there is a website using WordPress or not, they just need to look for source code.
Here is the process need to be applied:
1) Open the website url in your browser.
2) Press Ctrl + U, you will see the code editor. Even the firefox users can use Firebug and inspect the code.
3) Search for the pattern -- /wp-content/ or /wp-theme/ or simply theme or WordPress.