adli197, by going to the page Your Service and Support Coverage and having your iPhone serial number accepted and displaying what's available let's you know have Apple iPhone.
Is there something different about your iPhone that makes you question its authenticity? If so, any time you are near an Apple Store or Apple Authorized Service Provider you can have your iPhone examined. Just let the technician know and they will be able to let you know. Or you can use this link to see locations: Find Locations - Apple.
If there is anything else we can help out with, just let us know.
Take care!