Not a single secret involved. Nor any simple answers. What is involved is measurement by the taster of structure and balance, that ideally taken together with general knowledge of the variety or blend and the track record (that is to say, past developmental history) of the wines of the winery in question. As sometimes I also like to say, three other factors, those relating to the taster and, in this case, the person making the predicition: experience, experience and experience. And, if the absolute truth be told, even then there is a certain amount of following one's "gut feelings" (ideallly of course, those based on the experience to which earlier referred)
As to tasting 6, 12, 18 hours or days later, forget that! If one cannot make one's predictions after returning to a wine after 30 minutes he/she will not be able to make those predictions after any period of time whatever. Keep in mind as well my dictum (mine and not agreed to by all) that no wine should be held responsible for its qualities more than 12 hours after opening.
Of course keep in mind that predicted drinking windows also have to have a track record. That is to say, the critic has to demonstrate over time his/her ability to make such predictions with accuracy.