First
Refer to your planning and identify the main teaching objective for the lesson or sequence of lessons for the pupil or group of pupils. Locate the section of the maps for this objective. Find which step it is on or near to and look at the Examples of what pupils should know and be able to do. This will help you to confirm that the work is at the right level. The maps describe the progression in each strand. The objectives are drawn from the Framework for teaching mathematics but do not list all possible objectives. You will need to make your own professional judgement when using the maps.
Next
The maps will help you to decide whether the pupils have understood the mathematics by providing you with some Probing questions to ask them.
Finally
If you have a pupil or a group of pupils who clearly have not understood, then the What to do if pupils find this a barrier will give you some suggestions of how the understanding may be built up. Sometimes this will be to look at the previous steps, but more often there are teaching ideas and materials provided for that particular objective.