NumberQ
NumberQ[expr] gives True if expr is a number, and False otherwise.
NumberQ[expr] returns False unless expr is manifestly a number (i.e., has head Complex, Integer, Rational or Real).
NumberQ[Infinity] gives False.
NumberQ[Overflow[ ]] and NumberQ[Underflow[ ]] give True.
See The Mathematica Book on the web: Section 2.3.5 and Section 3.1.1.
See also: NumericQ, IntegerQ, TrueQ, Complexes.
Further Examples