You'd have to check the numbers and letters on the chip itself. In the first line, the last letter of the line will determine if it is a 100 or 133 chip. A "B" denotes a 100 chip, while a "C" denotes a 133.
Edit: Here's an example of a 1.2Ghz AXIA 133 fsb TBird.
A1200AMS3C
AXIA0117MPMW
Y6277140038
The bolded letter is the letter that shows you what the default fsb is.