They have no way of knowing, and only on very rare circumstances do they ask. My daughter was always very short and tiny for her age--my husband would just pick her up before we got to the gate and carry her in--no one asked and no one volunteered any info--this was at a local park, not WDW--she didn't go there until she was 6. I didn't feel guilty, because there was very little for her to ride anyway. I might not have done it for MK, because there really is a lot that the 3 year olds can do, and you don't feel like you are wasting your money.
If you are worried about it, buy the ticket if the child is eligible for the next age bracket, otherwise, bring the birth certificate if the child appears older than he/she is. It really is hard to tell nowadays---kids seem to be looking older and older at younger and younger ages <IMG SRC="/infopop/emoticons/icon_confused.gif" alt="confused">
Have a Wonderful day, Eeyore