Here is an outline of how to successfully string bend and how to gauge the difference between half tone bends and full tone bends.
1. Place your finger on the chosen fret and pick it.
2. Next, play the next fret up and that will be the pitch you want to bend the original note to.
3. Play the first note again and try to bend it to match the second note which was one fret higher. This is a half-tone bend. For a full tone bend, replace step 2 with step 4.
4. For full tone bends, play two frets up from the original note, and this is pitch you want to bend the original note to for a whole tone.
5. Now that you know how to do both, try and alternate between performing half tone and full tone bends and releasing the string back to the original position. After some practice, this technique will be mastered and is a useful addition to the arsenal of guitar techniques which create expression during lead playing.