The hotlink is more usual than we can imagine, there are people who hotlink images without being aware of it and others do it in a deliberate way.
After several years developing web pages I have experienced both cases:
There is no effective way to quickly detect if we are being consumed bandwidth by hotlinking:
RewriteEngine On RewriteBase / RewriteCond %{HTTP_REFERER} ^http://(.+\.)?domain\.extension/ [NC] RewriteRule .*\.(jpg|gif|jpeg|png)$ ourimage.jpg [L]
In the upper code I have an example to avoid that the web domain.extension not take benefit of my images jpg, gif, jpeg and png. In addition I return an image to them that I choose (dissuasive) that puts: ‘This Web takes content without permission of our URL.
This case is not usually used because at the SEO level it can affect us in websites such as google images, which refers to the photos on our website. But for who want to choose this option I provide you with the .htaccess code:
RewriteEngine On RewriteBase / RewriteCond %{HTTP_REFERER} !^http://(.+\.)?ourdomain\.extension/ [NC]