How to know whether a visiting professor will charge the university to present a lecture to academic staff?
The usual way is to state explicitly in your invitation which costs you are going to cover. For example:


  Dear Professor X
  
  I would hereby like to formally invite you to give a seminar talk at our university on a topic of your choice. We will of course provide you with a hotel for your stay as well as cover your airfare and all other reasonable travel expenses.
  
  Sincerely,  Y


This way, it is clear to everyone what exactly you intend to pay for. If Professor X wants anything else, he will have to ask for it explicitly, and you can then choose whether you think that you want to pay for that as well or not, but at least you put out there what you expect to pay for.