Nagios allows you to verify whether an FTP server is listening for connections by using the check_tcp command. This plugin is identical to check_tcp, with the difference that the port is optional, and by default a valid FTP welcome message is expected.

check_ftp -H host [-p port] [-w <warning time>] [-c <critical time>]
[-s <send string>] [-e <expect string>] [-q <quit string>] [-A] [-m <maximum bytes>] [-d <delay>]
[-t <timeout seconds>] [-r <refuse state>] [-M <mismatch state>] [-v] [-4|-6] [-j]
[-D <days to cert expiry>] [-S] [-E]

The port argument can be omitted and defaults to 21, or 990 for SSL based connections. A sample command definition for checking FTP accepting connections is as follows...