Assessment is collecting information about learners´ performance , progress or attitudes to make judgments about their learning.
Formal assessment through test and examinations and Informal assessment during a lesson by monitoring, listening and observing while they are doing ordinary classroom activities. Firstly we have to think in the reasons for assessing learners, then how to assess and the methods we are going to use.
For further information revise these videos:
https://www.youtube.com/watch?v=1UjiHA3g39g
https://www.youtube.com/watch?v=3wiIIfjD7RY
https://www.youtube.com/watch?v=_WDfT46nijA
Reference:
Spratt, M., Pulverness, A., & Williams, M. (2012). The TKT Course Modules 1,2 and 3 (2nd ed.). New York: Cambridge University Press.