programtically determine port routed

[gomer]

I have a client server application working but I need to know how to verify a port is open and routed to the client machine.
Basically a small network diagnostic to make sure a port is routed correctly if a router is used in a network.

edit for clarity - I am using a TCP sockets server / client and writing in VB 2005

I want to be able to verify a network connection on a specific port is available before streaming a file to them, but I want this to be a user defined port. so user enters port 2500 as the port to use and the client would ask the server to verify the port is open and can receive data on that port even if inside a network.