The main problem with this question is that different Creationists have different theories, which makes it hard for me to generalize. Also, some Creationists emphasize their opinions about Evolution to the point where their alternative is not fully spelled out.
The straightforward idea that "God did it" qualifies as several things, but it does not qualify as a scientific theory. This is because it is fully compatible with any evidence whatsoever.
However, Scientific Creationist theories usually do contain elements which are falsifiable. This is because they take the position that God only initiated or controlled certain events: other events took their natural course. For example, many Young-Earth Creationists argue that all fossil-bearing rocks were laid down during the natural course of a flood initiated by God. If these arguments are firmly central to their theory, then (and only then) is their whole theory falsifiable. It can be tested.
However, many organizations, including religions and the U.S. Supreme Court, have stated that Scientific Creationism is religion.