Just look at the serial number on the chip.
AFAIK, when people talk about the core being different than the nominal speed of the chip they are talking about the Slot-A type CPU's where the serial number of the chip on the plastic case indicates, say, a 700, but when they open up the case they find it's a 900 per the markings on the chip itself. If you have a Socket-A or "flip chip" type chip, though, you just have to look at it.