When they realize that a big proportion of your clickers aren't buying anything, anywhere... if they have access to this data.
I guess they can have feedbacks from clients on where they obtain their sales, and if on your site they are really low, or non existent, they will surely compare it with the typical profile of the people visiting sites similar as yours. Then they will just need to reach a critical value of abnormal behavior to claim with confidence that you are hiring a click farm.
this question cannot be answered precisely as
- As GdD said, it is counter-productive for Google to tell
- It certainly depends on the type of site, the time, the type of visitors (and their buying habits), the number of visitors per month, and the few elements that click farms may publicly post (hours, duration of the click sessions, global localisation of IPs -do they match geographically with similar site's customers?-, how many of the clickers aren't on other cookie-enabled Google services compared to the average, etc.)
The only advice I can give if you decide to hire such services is:
- If you still do it, be clever. Do it in a nefarious way that doesn't draw too much attention. Remember, you are still under the threat that the click farm may operate unstealthily and/or get busted, bringing down all its clients in the process.
PS: This is merely logical reasoning. I neither work for Google, nor do I have a stake in the click fraud industry =)