We're going to explore you some ways to detect site uses Drupal CMS. All of these suggestions wouldn't work on all Drupal sites, but taken together they should give you a clear answer.

If you look at a website such as WhiteHouse.gov, there is no way of telling if it's built Drupal. The design of a site is completely independent from the platform it uses.

1. Using Source Code

The most reliable ways to show if a site is using Drupal is to check the source code. In the source code, check to see if important files are being loaded from the /sites/ folder.

So if you're in Firefox you can use the cool keyboard shortcut (ctrl + u), or in chrome just right click and view page source to see the html markup.

When viewing the page source code, do a find (ctrl + f) for "Drupal".

2. Visit the User URLs

One common identifier of Drupal sites are the URLs for user pages.
The URLs /user/ and /user/password and /user/register/ are commonly used to allow people to login, recover their password or register.

3. Use this websites

Use the website http://isthissitebuiltwithdrupal.com/ to check site is build in Drupal or not.

That site gives very clear answer