nmap -T2 -O -P0 -sV -v -v <router's IP>

This will generally identify any machine you scan, as well as give you service/daemon versions. When you scan the router... Lets say it's a Linksys WRT54G... check the Linksys website. You should get a bit of information on it.

NMAP is required for this. Unix/Linux/BSD helps due to the manner in which hardware access is handled, but Windows nmap can scan just fine for your purpose.