Well, in my opinion, some kids make it very obvious very early that they are gifted. Some kids don't. And I don't think it's as simple as Ruf's levels. I don't think there's an algorithm that can tell you conclusively whether your preschooler is gifted. I don't even think the WPPSI will tell you that conclusively. But that doesn't mean that you can't know in some cases.
Now, whether you can know the LOG of your child in the preschool years is a bit sketchier IMO. So much depends on personality and exposure. I think it is possible to identify some HG+ preschoolers, but I think that is pretty rare. I have called my DD "likely HG+" because I don't think she's so, so advanced that it is conclusive at this point (though there are areas in which she performs at the level of a child more than twice her age). I don't think I'd think it was conclusive unless she were doing college-level work.
As far as whether gifted kids can "fizzle out," I guess that depends on your definition. I don't think that gifted kids become neurotypical. Of course they can lose their ability to perform at an advanced level due to physical issues, or they can lose their desire to perform at an advanced level due to emotional issues.
