If you are running linux on your xbox, you may well have the cromwell bios on your modchip, a nice quick way to determine the version of your cromwell bios is to use raincoat to read it, then grep the bios image for version strings:
(On Xebian:)
su -
apt-get install xbox-raincoat
raincoat -r /tmp/biosimage
strings /tmp/biosimage | grep -icromwell
This should give you something like:
Cromwell 2.40