Using the 'Model No' as found on the sticker under the unit (NOT the FCC sticker!):

It Says: You have:
WRT54G G v1.0
WRT54G ver1.1 G v1.1
WRT54G ver2 G v2.0
WRT54G v2.2 G v2.2
WRT54G v.3 G v3.0
WRT54GS GS v1.0
WRT54GS v1.1 GS v1.1
WRT54GS v.2 GS v2.0
Anything Else Use the serial #

 

Using the first 4 digits of the serial #:

It starts with: You have:
CDF0 G v1.0
CDF1 G v1.0
CDF2 G v1.1
CDF3 G v1.1
CDF5 G v2.0
CDF7 G v2.2
CDF8 G v3.0
CDF9 G v3.1
CDFA G v4.0
CDFB G v5.0
(Runs VxWorks!)
CGN0 GS v1.0
CGN1 GS v1.0
CGN2 GS v1.1
CGN3 GS v2.0
CGN4 GS v2.1
CGN5 GS v3.0
CGN6 GS v4.0
CGN7 GS v5.0
(Runs VxWorks!)
SJH0 WTR54GS v1.0