Contact other parents. Parents who have gone through the experience of helping preschoolers with cognitive development can be an excellent resource. The parent will likely give honest advice and can explain what works and what does not. The parent, having gone through it, will have a different perspective than an academic person.
Contact a child psychologist. A child psychologist is an expert in a preschooler's cognitive development. She knows exactly what the child should be learning how to do, where the child is (in his own development) and what learning needs to take place. The psychologist can give an expert opinion on the best activities to do with the child and why they are important.
Read literature. There are many books on cognitive development, written by parents, teachers, doctors and other experts. The books will give examples of the best activities for children to strengthen specific skills. For example, a child may be good at visual perception but struggle with language.
Interact with the child. The best way to help a child with cognitive development is by interacting with the child. Observe the child in different environments and have the child interact with different objects. Talk to the child and explain things for him. For example, give the child three blocks and ask: "How many blocks do you have?" Or ask: "What color are the blocks?"