I've done extensive research on Pre-post favs v SP favs, for preceisly that reason (you often don't know what is fav til it's too late) however I'm told that there are bookies who will accept wagers on the un-named favourite if that's the way you prefer to go.

My reasearch shows however that there's very little difference if any in the long run backing the pre-post fav instead, usually it's the same horse (some win some lose about 30% in both cases,) sometimes not and it wins sometime it loses .

Boy I managed to make that sound difficult, what I'm saying is that the SP fav wins about 30% of the time, and the pre-post fav. wins about 30% of the time though the selection will differ sometimes. Phew!

But taken overall theres very little in it, so to make life easier if I'm operating a mechanical plan I'll use the pre-post fav so my bets can be placed well before racing if necessary.
A word of warning though ALWAYS use the same source of the pre-post fav as publications differ.