It might help to find out what their general views about transgendered people are beforehand, and then you'll have some idea of how they would react if you told them.
Of course it's one thing to have a certain view when it relates to the outside world but, for good or ill, that doesn't always carry over if it's your own child. So it's not infallible, and you shouldn't assume the fact that if they're accepting and seem supportive of transgendered individuals, they would automatically transfer that acceptance to you if you tell them... but it would be an indicator.