Forcing popup when coming from external websites (e.g google)

I work for a restaurant guide in the NW of the UK and we basically have featured restaurants (the ones who pay a monthly sub) and non-featured restaurants (the ones who get a basic free listing). We were looking to get some kind of popup appearing on the non-featured restaurants, but only if they came from an external site, e.g Google. So the steps would be:1) User goes to google and puts in "name of unfeatured restaurant" 2) Our company comes up with the basic listing of that restaurant3) User clicks the link but is then presented with a popup of featured restaurantsBut, we would only need the popup to appear if the user has come from an external site. So if they were they came internally from our website the popup wouldn't show.We use ASP at work and I was thinking it may possibly need some kind of a variable being passed through, or maybe something to do with cookies but I'm not an expert on the ASP side. Hope someone can help on this :)
 
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