It is most common in guitar tab to see the letter h representing a hammer-on, located within the tablature between the original fret, and the hammered-on fret. So, if you were to see 7h9, you would hold down the 7th fret and pluck/pick the appropriate string, then hammer-on to the 9th fret without re-picking that string.
Occasionally, you'll see the ^ symbol used for a hammer-on (eg. 7^9)
Sometimes, in more formally printed guitar tab (like in sheet music books or guitar magazines), you'll see hammer-ons written as "slurs" (see above), with a curved line appearing over top of the initial and subsequent hammered-on notes.
