Unfortunately iOS does not currently provide a way for you to see your device's IDFA (Identifier for Advertising). There are some free apps on the App Store that will display your IDFA. Here are a few we've used that appeared to give the right information at the time we tried them (note that we don't have a relationship with these companies and thus can't vouch that these apps aren't malicious, or that they will give the right information in the future):
Note: If you are interested in your IDFA because you want to request that a company look up and/or delete your personal data, do not use the "Reset Advertising Identifier" button in your Settings app until you've written down, taken a screenshot of, or otherwise remembered your current IDFA. The "Reset Advertising Identifier" button gives you a brand new IDFA, but companies will have your personal data recorded under your old IDFA and won't be able to find your data if you don't know what your old IDFA was!