It is a continual process. The nature of primary teaching means that at the start of each session there is a starter activity, which will focus on prior learning, often done as a class. Then some teachign takes place,w ith work related to it. At the end of the session there is a plenary which checks, as a group, understading.
A good teacher will use these time to check the classes understanding. He/she will be able to pick up on who has and hasn't understood, and will make provisions for that in their planning for the next session.
The work set, related to the teaching, will also be used to check individual understanding of key concepts.
Lessons will build on one another, so the learning is not just based on one lessn but over a series of lessons and period of time. Key topic areas are revisited throughout the year and ver more than one year, so again, more checks here too.
Teachers keep records of what akes place.
Also generally every half term there will be some form of informal assessment. Often children won;t be aware that this is happening, if done well, but the teacher will gain excellent observations of the children through it.
Also, more formal assesments do occur - once a year, sometimes more, in many junior schools.