The Emergency Measures of the Ministry of Health states a general obligation to wear a face mask. This is monitored by municipal and also by state police forces. You can get a penalty up to 10,000 CZK if you fail to comply, however, the police can give you only a warning in case of a less major offence. The penalty is payable immediately, or later on during an administrative procedure.
If you fail to comply with a quarantine or a self-isolation, it can be treated as a criminal offence of spreading a contagious illness.
Police jurisdiction to monitor the public of wearing face masks is stated by a Special Law. Penalties are stated by the Act No. 258/2000 Coll., on the protection of public health (§ 92n, paragraph 1b of this Act), and by the Act No. 250/2016 Coll.,on the liability for offences and proceedings concerning them (§ 91 of this Act).