There's a plethora if network scanning tools out there. My two favorites are nmap and Angry IP Scanner. For the sake of simplicity, I'll demonstrate using Angry IP Scanner.
Specify the range you want to scan and then crack open the settings:
I like to run 100 threads to speed things up. You might get some wonky results if you go to high depending on the equipment you're running, so be careful and stay moderate if accuracy is of importance:
It can be of value to add a few common ports if you're looking for a specific device. I always run 80 and 443 to look for any machines serving HTTP. For me, it's very handy when looking for printers and routers:
Lastly I like to see only responding hosts:
Why can't I see everything?
There's a few caveats to scanning. It's not cut and dry.
- Windows 7 machines do not have ICMP ping enabled by default and may not show up
- Anything behind NAT is going to be "invisible"
- If you absolutely have to find everything, try using an nmap intense scan
- Nothing is guaranteed, you might not find everything
If your second router is performing NAT, which it likely is, you're not going to be able to scan that network from the outside (without some trickery that is beyond my knowledge and use.)
And for what it's worth, nmap is a lot handier when it comes to specifying ranges. For example, I often find myself using 10.0.0-255.1 to find all the routers across my network.