Go to www.cigaraficionado.com and look on the navigation bar on the left side for the Counterfeit Gallery (look for a little picture of a Cohiba band under a magnifying glass).
Compare the legitimate bands to the ones on your smokes and also check some of the known fakes for any matches. Also, was the Republica de Cuba tax stamp on the box, unbroken, before you bought it? If not, then it was most likely a counterfeit. Also, did your Cuban friend buy them from an actual shop, or just some guy on the street.
Use a search engine and check for some other sights that could help you see if the box is legit, and also some places to help you figure out if the tax stamp is real or not.
One more thing: If what you bought are Cohiba Esplendidos in a box with a clear glass top, they're fake. That's one of the biggest counterfeit tricks going on in the US and abroad with Cuban cigars right now.