In other words, jperez2009 was either running something on the background that raised the current CPU freqency or he has overclocked the lower limit above the default 250 MHz.

Back to your question, I doubt you could tell a fake N900 by dumping /proc/cpuinfo. I mean, there is no (longer?) such (a) thing as a completely fake N900, only a lot of refurbished devices that are being sold as "new". A complete fake (a device that looks like an N900 but has completely different electronics inside) would most likely not even have a terminal in which to type 'cat'.