You set up an activity for a child, an obstacle course running around the garden, jumping and hopping, leaping and climbing... the child enthusiastically accesses the activity.
You set up another activity for the child counting leaves you have collected from the garden... the child chooses not to access the activity and goes for a run around the garden instead, kicking leaves around and practising jumping on the hopscotch.
How do you know the child was interested in the activities you set up? What do you know about the child that tells you their likes, dislikes, interests, learning styles, things they do at home etc.
By observing the child, listening and watching and talking to the child and their family, you can build up a picture of what the child likes etc.
By evaluating past activities the child has accessed and enjoyed you continue building up the picture.
Then you can provide a learning experience which makes the child want to join in and be enthusiastic about.
The child above likes movement and is learning from being physically active - a counting activity with conkers is not interesting to him... however a counting activity with the hopscotch might well have the same intended learning outcome but would better meet his needs
You have learned this through your evaluations