Using Perl/Net::SinFP (sinfp.pl) for OS fingerprinting

SinFP is a new approach to OS fingerprinting, which bypasses limitations that nmap has... Nowadays, with the omni-presence of stateful filtering devices, PAT/NAT configurations and emerging packet normalization technologies, Nmap's approach to OS fingerprinting is becoming to be obsolete...
SinFP uses the aforementioned limitations as a basis for tests to be obsolutely avoided in used frames to identify accurately the remote operating system...

See this security tools' webpage.