In most of Prague's restaurants, service is not included.
Tips aren’t usually left by guests on the table. Should you wish to tip staff, add around 10% onto the bill or round it up. It’s up to you what sum you tell the waiter you wish to pay when he brings the bill. If you say nothing, you will receive the exact change.
If you say the word ‘děkuji’ when the waiter is giving you the change, he will understand that the remainder has been left as a tip.