FreeQ[expr, form] yields True if no subexpression in expr matches form, and yields False otherwise.
FreeQ[expr, form, levelspec] tests only those parts of expr on levels specified by levelspec.
form can be a pattern.
Example: FreeQ[f[x^2] + y^2, x^_] .
FreeQ looks at the heads of raw expressions, testing whether those heads match form.
See The Mathematica Book: Section 1.8.5, Section 2.1.5 and Section 2.3.5.
See also: MemberQ, Count.