Hope for the best but plan for the fact he/she may not measure up. Don't give up if they measure short at test track, definately try again at say splash mtn. I have found the measurements can vary greatly.
We have had several trips with kids close to one of the cut off points, 40, 44 or 48" so I know its tough. Regular but slightly thicker soled sneakers are ok but I have seen CM's ask for obviously thick soled shoes to be removed.
Ask the child to practice standing straight - kids tend to naturally shy away from the stick, esp since it looks as if it's going to whack them in the face! Tell them to stand straight (no tippies) and stop that stick!
Be prepared for the possibility that even if he/she measures up at the entrance a 2nd CM may measure at boarding. If they don't measure at boarding they don't ride - this has happened to us a few times and it's tough. We have also had my dd measure tall enough for Space Mtn the first few days of the trip and suddenly "shrink" on the third day.
Have a backup plan in case you are denied, hit a favorite ride again, share an ice cream or indulge him/her in an extra run down the pool slide later in the day.
The CM's have visual guides that they can look at to spot boarderline guests. I know at a local park the queue railings are the same height as the height restriction so its easy to watch the line and decide if anyone needs to be measured. I don't know what the visual cues are at WDW but I have been told by a CM that they exist.
Remember that the height restricted rides are a small % of the overall Disney experience. Even a child under 40" will be able to do most of what is offered
TJ