[ventegus]

They check for client IP. True Googlebot always comes from 66.249.*.*

[seanw444]

Yeah I was like "surely it can't be that easy." So I went to try, and no, surely it is not.

[iggldiggl]

Sometimes you can get lucky though – I know at least one forum that requires registration even just for viewing posts, but lets Googlebot through and only checks the user agent.