Is it possible to detect if my visitors are coming to my website via a Google Adwords Ad?
It would like to change the telephone number for those users only, that way I can measure the success of my adwords campaign with the number of calls.
You can use the special utm_ query string parameters mentioned here: http://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=156114
Then switch your phone number out based on (for example) utm_campaign
If you use the auto tagging feature (on by default) the parameter to look for is gclid: http://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=55596
On the server you need to grab the value of "HTTP_REFERER"
Also, Google Analytics should be able to do this for you, out of the box.