I'm using node.js with express.
I want to present a user arrived from google adwords campaign a specific banner. I can use req.get('referer') and see that it's google.com but how can I know it's from a compaign? and how can I test it locally?
You should add tracking params to your Google AdWords campaigns, e.g. ?source=google-adwords. You can then check for the existence of this param. This is what tracking systems like Piwik do.
Google AdWords allows to pass dynamic param values named value track params to gain deeper insights about what keywords and ads your traffic came from.
You could link Google AdWords with Google Analytics and automate tagging of your ads, which will automatically add a param named gclid to all of your Google ads links. You could then check for the existence of this param.
Referrer should be the last way to go, as it is not very reliable.