Similar to the hammer-on, the pull-off is generally represented by the letter p in guitar tab, appearing between the originally fretted note and the pulled-off note. So, if you were to see 9p7, you would fret and pick the 9th fret, then without re-picking pull off your finger to reveal the note behind it on the 7th fret. Occasionally, you'll see the ^ symbol used for a pull-off (eg. 9^7).
Sometimes, in more formally printed guitar tab (like in sheet music books or guitar magazines), you'll see pull-offs written as "slurs" (see above), with a curved line appearing over top of the initial and subsequent pulled-off notes.