See timestamps in iOS 7's Messages app

The Messages app in iOS 7 doesn’t introduce much in the way of new functionality from iOS 6—though it does sport a dramatically different look.

While iOS 7 shows timestamps in between conversation breaks, you might sometimes want to do know precisely what time a given message arrived. There’s an easy way to find out:

Simply swipe across a conversation in Messages from right to left. As you drag, you’ll reveal per-message timestamps. They slide away as you release your swiping finger.

Source: Mac OSX Hints