How old he or she is ? I have someone in my house, and the very first thing to observe, is to observe the communication between the child and others. Does he responds to everything that you ask ? Does he reacts to everything that he sees around ? 

If the child is responding in an engaged way with you in a normal way on everything, then there is nothing to be concerned. However, if he only interacts to things of his interest, and does not listen or react anything else, then he may need to be watched out. And more ever, if he is busy doing something on his own, without listening to you, and you try to intervene, then they show anger and frustrations. So its like they like to do what they wish to do, but they don't respond the same to you . In that case, you should show him to a doctor for further check up. 

And if your child is less than 2 years old, then it will be highly impossible to derive a conclusion. 

So the best way is to engage as much as possible with the kids, because the best treatment that can bring change in the child's behavior is within 5 years.