What happens if you do not have a valid ticket or ID?

The inspectors normally ask you to pay the fine on the spot directly to them. If you do not wish to pay, they are entitled to ask for your ID on the basis of which they issue a summons for payment. If you do not have an ID, they may call the police in order to determine your identity.
You have 14 days to pay the fine. If you pay directly on the spot, or within 7 days from the date, the fine is lower.