As an English language teacher how do you know that your students (have or have not )understood your lecture or the lesson that you have delivered?

ask'em questions

I have a variety of ways l can employ.Some among these would be to ask oral questions to the pupils.I can also do ''cold calling'' where l pick on any pupil and ask them a question.I can give a summative exercise and at the end of it all,these will point to how much my pupils have understood the matter at hand.

I can understand it from their facial expressions, or when I ask them and do not know the correct answer or the answer is not related to the lesson or topic ...

It's recommended to give students short exercise as a means of feedback to measure their understanding and also I can observe this myself through their expressions.

Formative assessment

I, simply, test them

by asking question and assement the interaction,then exercises

A simple question and answer session at the end of a lesson/lecture will tell you straight away if "learning" has taken place. Over a longer time period their final exam results will definitely tell you but by this time of course it will be too late! Therefore a series of small tests conducted over the period prior to the exams, coupled with an ongoing assessment of their course work will determine if there is a need for remedial action to be implemented.

I always try to keep my students engaged during the study session. This methodology helps me in 

1. keeping the students busy.

2. avoiding disorder in the class.

3. grabing full attention of the class.

4. observing the response of the students.

5. encouraging students to put any question related to the topic on the spot.

6. refurbishing brain of the students according to the topic.

To cross check my efforts I always ask my students to ask if there is any question in their mind. Moreover, to verify it I analyse their face expressions and I categorically ask a doubtful face about his conception.