Two fundamentally different systems
Barrier tolls
You stop, take a ticket or pay directly. Payment is settled on the spot and nothing follows you home. Card acceptance varies, so carry some cash.
Free-flow gantries
Cameras read the plate at motorway speed and there is nothing to stop for. Payment happens afterwards, either through a transponder in the car or by post-trip billing to the rental company.
Find out what is in the car
Ask at handover whether a transponder is fitted, whether it is active, and how usage is billed. A device that is present but not registered generates unpaid-toll penalties rather than tolls.
Where a daily transponder rental fee applies, check whether it is charged for every day of the rental or only days you use a toll road.
Country-specific systems
Some markets run their own schemes with their own rules — Dubai's Salik is a common example for visitors, and it is billed entirely after the fact.
Where a destination has a distinct system, read the dedicated guide rather than assuming the general pattern applies.
Unpaid tolls become fines
A missed free-flow toll does not stay a toll. It escalates to a penalty, which the rental company pays and passes on with an administration fee.
This is the most common way a modest toll becomes a significant charge weeks after the trip.