Go to Facebook Like Button page: https://developers.facebook.com/docs/reference/plugins/like/, scroll down to "Step 1 - Get Like Button Code", enter your website address or your web page URL in "URL to Like (?)" field then press enter.
Now, on the white area you will see the number of likes of the URL you entered ... So simple!
There are also some other ways to know now how many likes your page / site got on facebook - like embedding the Facebbook Like button to your page or using some online tools etc. but I found this one as the simplest and easiest.
