Generally if they think the child is over 3 they will direct the question to the child... you know get on their level and say "so how old are you"... kids are more than happy to get a great big smile and say "this many"... O the cute things we teach our kids... or they will ask what year the child was born, catch you off gaurd, if you are telling the truth you will answer without hesitatition, if you are not you will stumble.
It is not a bad idea to have a copy of the birth certificate as mentioned, but it is usaually not necessary, the CM's are trained to take your word for it, unless the kid is obviously like 8 years old and shoved in a stroller (it happens)