nmap -sV -O (your target IP here)
You can fingerprint more that just AP's. In fact, just about any device connected to a network will do. The above command will portscan your target to see what services are running. Afterward, it will probe the services and try to determine the exact version. Based on the results, you can make a guess what operating system is running on the target machine. Some services and open/closed ports are specific to Microsoft Windows (fingerprint) while others are more specific to linux. It really depends on the configuration of the target host you're scanning, and based on the scan results you make a "guess" about what OS the machine is running.
There are a number of fingerprinting/port scanning tools, but nmap is the most popular and widely used.