Give the child some simple directions and monitor whether or not he is able to follow them.
Observe his fine motor skills, paying close attention to such things as walking, running, jumping, skipping and picking up items. Note whether or not he seems to be at the same level as other children his age.
Start a conversation with the child, noting whether or not he is able to find the correct words, pronounce words correctly, etc.
Read a book to the child, noticing his ability to identify shapes, colors, numbers and letters.
Observe whether or not the child is able to sit and pay attention for short periods of time or if they become restless quickly.
If you notice any of these symptoms, consult with your pediatrician who may refer the child to a trained professional for further evaluation and treatment.