Here’s a quick, dirty, and not 100% reliable command to tell you which IPs in a range are in use:

nmap -sP 192.168.1.1-254

The above example will scan the range of 192.168.1.1 – 192.168.1.254, and will give you a result something like this:

Host (192.168.1.1) appears to be up.
Host (192.168.1.2) appears to be up.
Host (192.168.1.14) appears to be up.