Concept checking is normally achieved by the use of a set of questions designed to ensure comprehension of the target language, raise awareness of its problems, and to indicate to the teacher that the learners have fully understood.
The question 'Do you understand?', or the remark 'OK?' do not achieve any of these aims, and are unlikely to receive a truthful answer from all the learners. Concept questions are one way of checking understanding, but are often used in combination with other methods, often visual, depending on the nature of the target language involved. Here are some other methods: