needs help to implement check taht some port in using

Hello! I'm try to build own simple ftp server in go (but backend
connects to rackspace swift server and store file in the cloud). If
client send PASV and EPSV commands server need to spawn listener on
his side and send to client ip and port. I need to check that port is
not used by any application and if it used, random set another port
and check again...
What is the simplest and fastest method to check - is the specific
port in listened by some one?
Thanks!

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Vasiliy Tolstov,
Clodo.ru
e-mail: v.tolstov@...
jabber: vase@...