(Note: the screen shots here are from a Samsung Galaxy S III smartphone, but the method applies to all Android devices--at least the ones that have not tinkered with [i.e., customized] the interface)

First, touch the menu button on the bottom left corner of your phone (i.e., the button that looks like part of a page with two lines on it). Then, from the menu that pops up, select "Settings."

On the Settings page, scroll all the way to the bottom and tap "About Phone" (it may say "About Device" or something else if you're not on a phone).

About midway down the "About Device" page, you should see "Android version." If you have 4.1.1 (or greater) you have "Jelly Bean" (the newest version of Android as of this post).