how do I know if my device has warranty?

Check Your Service and Support Coverage - https://checkcoverage.apple.com/ - "Review your Apple warranty status and eligibility to purchase additional AppleCare coverage.." Requires a hardware serial number.

http://www.apple.com/legal/warranty/products/ios-warranty-document-us.html - "IMPORTANT RESTRICTION FOR iPHONE, iPAD AND APPLE TV SERVICE. Apple may restrict warranty service for iPhone, iPad and Apple TV to the country where Apple or its Authorized Distributors originally sold the device." <-- Read this as they almost certainly will refuse to do warranty coverage except in the country where you bought it.