How do I determine if an origami crease pattern is at the intermediate level of difficulty?
While there is no surefire way to know, some cues may indicate a simpler or a more complex model:


If it's a box-pleating model or the CP is otherwise based around a grid, the size of the grid is usually correlated to the model's complexity. As a rule of thumb, something that uses a 16×16 grid will usually be simpler than something based around an 48×48 grid.
The complexity of line intersections in the CP may indicate how many layers of paper there are in different parts of the model which in turn may indicate the complexity to fold. So intersections where a larger number of creases meet may indicate a more complex model. There will, of course be exceptions, as a simple mountain fold over multiple layers may have the same CP (except for fold orientation) as a complex closed sink along the same line.
Look at the finished model. The amount of detail will often indicate the complexity. Bad surprises do happen, however.
Look around forums for people writing about the model — either the folders themselves or people commenting their work will often mention the model's complexity, especially if it is high. Don't be afraid to ask, either.
Get some experience by folding simple designs first. Even if you do not intend to fold tessellations in the future, they may be a good starting point into Crease Patterns since tessellations are often published in the form of CPs but at the same time the complexity of these CPs is usually much lower than that of animal model CPs. As you gain experience by folding simple Crease Patterns, you will become more proficient in judging the complexity of a CP.