You may need to know your iPhone or iPad's UDID (Universal Device Identifier) in order to give it to an iOS developer or to help you identify your device's iTunes backup folder. Here is a quick guide to help you locate your iPhone, iPad, or iTouch UDID, and to copy it to the clipboard if needed.
Step 1: Plug your iPhone into a computer, and select it in iTunes.
The computer you use does not need to be the one you sync your iTunes library with1, and you will not need to do any syncing or backing up to get the UDID (so you can dismiss any prompts to sync or interact with the iTunes library.)
In iTunes 12, you should see an icon for an iOS device in the top icon bar, click that to view the summary information for your iPhone2.
Step 2: Click on your iPhone serial number to display the UDID.
Step 3: (Optional) Copy the UDID for pasting elsewhere.
With the UDID visible, press Command+C (Mac) or Control+C (Windows) to copy it to your clipboard.
If you are using a different computer, or have never synced with iTunes before, you may see a prompt asking if you want to setup the phone as new or restore from a backup. Choose "Setup as New." Despite the horrible phrasing, "Setup as New" doesn't wipe your iPhone or anything terrible, it just adds the phone to iTunes as a new entry. ↩
I am going to refer to iPhones, iPads, and iPod Touches as "iPhone" from here on out to save some words. ↩