reuploading the firmware should also work, it usually just reimages the system partition. if you don't know if you trust that you can also go to dd-wrt, openwrt or tomato and once you see that running you know the system partition has been reimaged and should be fine.
To a point. It's possible to persist a backdoor on these routers beyond a firmware upgrade. If you think you're a big enough target for someone to actually install a backdoor like this on your systems, I'd be happy to take a look.
