The most common is the click-through tracking options that are used by many publishers. If a publisher does not offer this you may wish to use a third-party tracking solution or even a proprietary solution. My opinion is to use a form of tracking on top of what a publisher offers. Once a campaign has run it's course, you then can gage the effectiveness of your efforts. The first is the clicks.
Clicks demonstrate the interest gathered from the creative used when campaigning.
The more clicks you receive then you possess a better chance of conversion. The next is the actual conversion. Converting those interested clickers into customers. If you receive 450 clicks and only 2 conversions then there is definitely a problem. This will help you determine where the problem is.