You could use a beacon directly in the HTML or CSS. Your HTML for example would look something like this:
You have to use the absolute URL.
Gets a little complicated past here and I don't have an example but beacon.php is going to serve a transparent .gif to all users, you could even set this for common image you want to serve. The script is also going to check the referrer and if it's not your website you can have it email you, log it etc.
The trouble with this is can easily be edited out but I guess that's going to be the case no matter what you do.
You're better bet is checking Google once in while for unique phrase that appears on your site.