By default, it's the apostrophe key.
Press it once, and you'll see a green target box surround the icon of the friendly nearest the centre of your screen. Press it again quickly, and the target box will shift to the next friendly. Keep jabing until the right guy (the one being attacked) is highlit with the box, then release the key. After one second he will get both a White text message and an audible warning to "check six".
The audible warning is a .WAV file on the target's own P.C. Thus, you can change it for another sound, in the same way you can change all other AH sounds. Most sound packs (Such as Mitsu's) include a new Check 6 sound. Mitsu's sounds quite special :-)
