After waiting a long time and you know for certain that the MacBook Pro / Air has zero remaining liquid within it, you’re certainly welcome to turn the Mac on yourself and see what happens. For most users though, the best bet is to wait until it’s dry, then take it directly to an Apple Store so they can determine if there’s any damage, and if so, what damage is done to what components.
If you’re very fortunate and act quickly, you may get away with no damage to the MacBook at all. Or maybe you’ll only end up with just a damaged keyboard, while the remainder of the components are fine. If liquid got the logic board or power system, the Mac is probably beyond a simple repair, in which case you’ll be out some serious cash unless you had a good insurance or an accidental damage policy on the Mac