How to politely enquire how serious someone is when they say "work for me as a postdoc"
I would follow up the verbal conversation with an email.  For example


  Thanks for the encouraging words the other day about a possible postdoc.  I wanted to check, were you talking about a funded position?  Where would I find instructions for submitting an application?


Depending on the vibes you got, you might want to start with something less formal, for example


  Thanks for the encouraging words the other day about a possible postdoc. I was wondering whether the position might be available for the upcoming semester.