Check current restrictions before you visit crag and be respectful to the nature. Most crags are located in nature protected areas, climbing there is subject to the consent of nature protection authorities and compliance with the conditions set by them.
Where to check:
Český horolezecký svaz - Czech Mountaineering Association: Ochrana přírody - Nature protection (czech)
Agentura ochrany přírody a krajiny ČR - Nature and Landscape Protection Agency of the Czech Republic: vyhledávač - search engine (czech)
Skalní oblasti ČR: Moisture conditions map, climbing restrictions map (czech)
or you can directly ask Czech Mountaineering Association on email email@example.com