Do creationists know that they can't prove their theory?
Actually, creationism doesn't even aspire to "theory" ranking, scientifically speaking.
While scientists cannot "prove" (barring time travel) that species x evolved from species y, they can monitor existing species and, over time, observe changes. In this way, they can prove that evolution *does* occur, even if they cannot go back and prove that it *did* occur. This, coupled with archaelogical evidence will be able to show a pattern of evolution.
Creationists got themselves a book.