Take a look at your router's config page to see what firmware it is using (usually 192.168.1.1, or go to System Preferences --> Network --> Ethernet [if using wired, and the address beside Router] or Airport --> Advanced --> TCP/IP and the address beside Router). If using DD-WRT, you will see the logo immediately. If using something like HyperWRT, on the top right of the page it should say "Firmware Version: v4.71.1, Hyperwrt 2.1b1 + Thibor15c ", for example. If it doesn't say that, you probably have the original firmware (and I'm going to assume you do).
I've had my Linksys WRT54GL since June 2007 and it works great, although occasionally I have to power cycle it. Nevertheless, this is one of the best consumer routers available and should last quite a while.