My suggestion: find people who have young children right now, or are about to. Volunteer to help them out if they ever need any assistance, and see if you can observe what the daily routine with children is actually like. (If you had younger siblings, you may already know a bit of this, but seeing it from the view of someone who's actually responsible for them is going to be quite different.) If you find good reasons for why you'd absolutely hate the experience, then that's some evidence for your preferences not changing (though having your own child is quite different from helping with another person's child), and if you realize that it doesn't seem that bad after all, then your preferences have already shifted and you'll know it.

Why is this a problem? Freedom of choice is a good thing to have. If you really want to commit future-you to the choices that now-you wants to make, there is a variety of surgical procedures that are moderately hard to impossible to reverse. But again, why do you want to give up future choice? [...] Future is uncertain. Your preferences will never be cast in stone and that's a good thing. Whatever you plans are, they should take into account the fact that things will (not may) change.