By paying attention to the language development of the kids in your family.
if the kid reachers 1.5 - 2 years and didnt start saying any words... then get that checked.
if the kid starts scratching their ear and is crying about it... get it checked.
if the kid is about 3 and doesnt use 3-4 word phrases... then get it checked.
Go to your areas mostaw9af, and talk to the doctor about it. they should refer you to the appropriate place.
Don't wait on this stuff, and as much as you can encourage the kids around you to learn and stop playing bloody PSP for the love of god.
this has been a public service announcement by the dude.