It's difficult to find a proper translation that would be
1)brief
2) and cover the meaning of "we are on to you".

Нам известно всё or Мы знаем всё

National Benefit Fraud Hotline - "Горячая линия" для сообщений о мошенничествах с пособиями (льготами)

You see, blawith, you have to be more specific and IDEALLY provide more context.
Here I understand that this hotline fights benefit fraud (i.e. situations when people fraudulently claim benefits or claim higher benefits than they are entitled etc.).

You need to specify the type of benefits (what's this system all about etc.). In Russian this "benefits" could mean a lot of things: "пособия" or "льготы", "выплаты", or even "пенсии" (old-age benefits or pensions) - go figure!

Cheers.

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Note added at 2 hrs 21 mins (2005-06-13 21:30:52 GMT)
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Possible options include:

Мы знаем всё.

Мошенничаете? Мы о вас знаем!


The problem is that if you translate it literally \"we know who you are, what you are doing and where you are\" etc, - why these \"we\" did not yet catch these \"you\" i.e. the fraudsters?