The beers that are most likely to be described as "hoppy" are the ones with the most bitter flavor or the ones that have fruity, floral aromas. Dark beers, like porters and stouts, are dominated by the taste of the malt component, so they would never be described as "hoppy".
International Bitterness Units (IBUs) is the scale used to measure the bitterness of a beer. It measures the isohumulone in a beer – this is an acid imparted from the hops. The scale starts at zero and the higher it goes, the more isohumulone that is present in the beer.