How to ensure spam email is being trained when using Android's k-9 email client

I have a standard Ubuntu Webmin/VirtualMin install running Postfix and spamassassin. How do I train the spam? I understand flagging it as spam on my phone just moves ito to a spam floder, and something on the server then watches this and logs it as spam, it that part I’d like to check is working because i’m sure it isn’t.