Your question is absolutely not stupid. The answer is no, we cannot make such a deduction based purely on Newton's laws.
The fact that the body in question moves along the incline and not into it (or away from it, for that matter) is a constraint which we are imposing by hand. Only after we do so can we use Newton's laws to calculate the normal force which must exist between the incline and the block.
As a general rule of thumb, there are two broad categories of things in Newtonian mechanics. First, there are dynamic objects with masses and accelerations, to which we can apply Newton's laws to determine how they will move. Second, there are constraints, which we impose when we already know (either from experience or by physically motivated assumption) how they will behave to good approximation. This can dramatically simplify the analysis, but we should be careful to remember that we're doing it.
The incline in your problem is an artificial constraint, in the sense that it constrains the motion of the body to be along its surface and does not otherwise obey Newton's laws. In principle, it is possible to treat the incline as a dynamic object which can be compressed and deformed, but (a) this is far beyond what could reasonably be done in an introductory course and (b) the assumption that the incline serves only to constrain the body's motion is a pretty accurate assessment of what actually happens under standard laboratory conditions - though if the incline were unusually squishy or the mass unusually heavy, this assumption might break down.