How can I check if my iPhone is covered under warranty?

Apple covers your iPhone under a Limited Warranty for one year from the date of original purchase. To check if your iPhone is still covered use Apple's online service assistant:

Click the link above and enter your iPhone serial number and your country. Apple may ask to see proof of purchase documents to verify your iPhone's warranty status before service is performed.

Check these instructions to look up your iPhone serial number.

I want to add that the alternative way is to use iPhone IMEI checker like
It is free and you can identify purchase date, warranty and contract status.