How to politely enquire how serious someone is when they say "work for me as a postdoc" I am a PhD student that is about to graduate. Sometimes professors whom I know personally (and/or have collaborated with before) will say "Come work in my lab as a postdoc!". 

However, when I approach them after a few days/weeks/months, they inform me they would like to hire me, but do not have any funding.

Personally I would rather they not have not said "Come work in my lab as a postdoc" if they weren't sure they would have the funding, but I understand their reasons for saying such things.

However, in future, how can I politely enquire how serious they are about how interested (and able) they are in hiring me?
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.