Nmap has a streamlined firewall filtering identification function that can be used to identify filtering on ports based on ACK probe responses. This function can be used to test a single port or multiple ports in sequence to determine filtering status.
To use Nmap to perform firewall identification, you will need to have a remote system that is running network services. Additionally, you will need to implement some type of filtering mechanism. This can be done with an independent firewall device or with host-based filtering such as Windows firewall. By manipulating the filtering settings on the firewall device, you should be able to modify the results of the scans.