Most motorways are equipped with ANPR cameras (Automatic Number Plate Recognition).
That means, cameras scan licence plates continuously and instantly compare them with the official e-vignette database.
Additionally, the Czech Police and Customs Administration are using mobile camera units, patrol cars with scanning systems, and handheld devices during roadside stops.
Related articles:
What happens if I drive without a Czech e-vignette?
Can I drive into the Czech Republic without a valid e-vignette if I avoid motorways?
Where can I check the status of my Czech e-vignette?
Can I drive to the first exit on the highway in the Czech Republic without a Czech e-vignette?
Do I need a payment receipt or print-out of the Czech e-vignette?