The serial number of the router should start with something similar listed below. You can find the serial number on the bottom of your router.

Here is a great chart:

»www.linksysinfo.org/foru ··· ?t=47124

Chart by jdepew

Here is another list:

WRT54G
CDF0 = WRT54G v1.0 / ADM6996L Chipset / CPU 125Mhz
CDF1 = WRT54G v1.0 / ADM6996L Chipset / CPU 125Mhz
CDF2 = WRT54G v1.1 / ADM6996L Chipset / CPU 125Mhz
CDF3 = WRT54G v1.1 / ADM6996L Chipset / CPU 125Mhz
CDF5 = WRT54G v2.0 / ADM6996L Chipset / CPU 200Mhz
CDF7 = WRT54G v2.2 / BCM5325EKQM Chipset / CPU 200Mhz
CDF8 = WRT54G v3.0 / BCM5325EKQM Chipset / CPU 200Mhz
CDF9 = WRT54G v3.1 / BCM5325EKQM Chipset / CPU 200Mhz
CDFA = WRT54G v4.0 / BCM5352E Chipset / CPU 200Mhz
CDFB = WRT54G v5.0 / BCM5352EKPB Chipset / CPU 200Mhz
CDFC = WRT54G v5.1 / BCM5352EKPB Chipset / CPU 200Mhz
CDFD = WRT54G v6.0 / BCM5352EKBG Chipset / CPU 200Mhz
CDFE = WRT54G v7.0 / AR2317 Chipset
CDFF = WRT54G v8.0 / BCM5354KFBG Chipset / CPU 240Mhz
CDFG = WRT54G v8.0 / BCM5354KFBG Chipset / CPU 240Mhz

NOTE: The WRT54G v5.0 has 2Mb/8Mb Ram, it uses VxWorks OS and is incompatible with 3rd Party Firmware.
For more info see this thread: /forum/remark,14563605

WRT54GL
CL7A = WRT54GL v1.0 / BCM5352E Chipset / CPU 200Mhz
CL7B = WRT54GL v1.1 / BCM5352E Chipset / CPU 200Mhz
CO61 = WRT54GL v1.1 / BCM5352EKPBG Chipset / CPU 200Mhz

WRT54GS
CGN0 = WRT54GS v1.0 / ADM6996L Chipset / CPU 200Mhz
CGN1 = WRT54GS v1.0 / ADM6996L Chipset / CPU 200Mhz
CGN2 = WRT54GS v1.1 / BCM5325EKQM Chipset / CPU 200Mhz
CGN3 = WRT54GS v2.0 / BCM5325EKQM Chipset / CPU 200Mhz
CGN4 = WRT54GS v2.1 / BCM5325EKQM Chipset / CPU 200Mhz
CGN5 = WRT54GS v3.0 / BCM5352E Chipset / CPU 200Mhz
CGN6 = WRT54GS v4.0 / BCM5352E Chipset / CPU 200Mhz
CGN7 = WRT54GS v5.0 / CPU 200Mhz
CGN8 = WRT54GS v5.1 / CPU 200Mhz
CGN9 = WRT54GS v6.0 / CPU 200Mhz
CGN9 = WRT54GS v7.0 / BCM5354KFBG Chipset / CPU 240Mhz

Thanks to linksysinfo