NumericQ[expr] gives True if expr is a numeric quantity, and False otherwise.
An expression is considered a numeric quantity if it is either an explicit number or a mathematical constant such as Pi, or is a function that has attribute NumericFunction and all of whose arguments are numeric quantities.
In most cases, NumericQ[expr] gives True whenever N[expr] would yield an explicit number.
See The Mathematica Book: Section 2.3.5 and Section 3.1.2.
See also: NumberQ.