Local inspectors could fine you up to 950 CZK if you do not have a signed pass or validated ticket during a trip or a stay within the compulsory ticket area. Please note, that only tickets punched in the yellow little machine are valid!
TIP: Find out which kind of ticket you need to buy to avoid fine by ticket inspector.
Ticket inspectors prove themselves by an inspection badge designated "Dopravní podnik hl.m. Prahy, akciová společnost - Ticket inspection" /Prague Public Transit Co. Inc. - Ticket Inspection/ and by a four digit ID number (see picture). They are also obliged to show you the service card with an ID number and photograph.
The penalty fare is 950 CZK; however, if you pay on-the-spot the amount reduces to 500 CZK (plus the fare). You should get a receipt confirming the payment of the fine (see picture).
TIP: Find out more about Prague city transport system.