The default key is the quote (') key -- typically next to the Enter key.
It will highlight (a box will appear around) the player closest to the center of your view. If that is not the player you want to Check Six, quickly hit it again and it will toggle through different players in your view. When you have highlighted the right player, just let it do it's thing -- it will send the check six message. If you check six the wrong player, it's usually nice to let them know you selected them by accident.
The Check Six key is particularly useful in furballs or areas where there are a lot of players using vox (especially if there is a very chatty player who does not to use any kind of brevity, like you will often find over GV spawn fights). If there is a lot of talk, it creates a lag/delay in the vox buffer. Ever get a voice check six 5 seconds after you got shot? That might be why. The Check Six button is quite useful if you like keeping friendlies alive. I even have it mapped to my stick.
Hope this helps.
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