fastest way to check if dir is a workdir

Hi,

I'm writing a little php-git-wrapper and would like to have a function
which tells me, whether a given path is part of a git checkout or not.

What would be the fastest commandline to check, whether the current dir
is part of a checkout? It may be assumed that the repository is at the
standard position.

Thanks a lot,
-- 
Thomas Koch, Software Developer
http://www.koch.ro

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