This site is arguably the most up-to-date source of information on the Czech motorway network and its regulations. To date it only mentions police checks. Never does it talk about automatic checks. Indeed the motorway vignette is a 2-part sticker. One part is to be stuck to the windshield, whilst the other is to be kept as a proof of purchase and is to be shown when being checked by police. The reasoning here is that it would be somewhat hard to show the proof of purchase to a camera driving by an automatic control.
Quoting from the linked site:
A proof of payment of the fee is a 2-part sticker. Part one of the sticker is to be placed on the inside of the windscreen. Part two of the sticker is to be retained.
During police checks, both parts of the sticker are required.
Indeed the rest of the internet seems to agree with this - for example here is another site vaguely mentioning police checks. It is therefore safe to assume that, to date, the payment of a motorway sticker for vehicles with a gross mass < 3.5t in Czech Republic is checked only when one gets stopped at a manned police control point. In other words, there seem to be no automatic controls for such vehicles. Lorries and other >3.5t gross mass vehicles however have their own automatic toll-payment mechanism, on which you can find more information here.
Having said this, at the time of writing the penalty for driving with an unvalid/expired vignette or without one are of (quoting from the afore-mentioned website):
Fine of up to CZK 5,000 or a penalty notice of up to CZK 500,000 in administrative proceedings.