NumericQ (Built-in Mathematica Symbol)

NumericQ[expr] gives True if expr is a numeric quantity, and False otherwise.

Numeric Quantities (Mathematica Tutorial)

Testing for numeric quantities. Pi is a symbol, so Pi+3 is not explicitly a number. It does however have a numerical value.

Putting Constraints on Patterns (Mathematica Tutorial)

Mathematica provides a general mechanism for specifying constraints on patterns. All you need to do is to put /;condition at the end of a pattern to signify that it applies ...

NumericFunction (Built-in Mathematica Symbol)

NumericFunction is an attribute that can be assigned to a symbol f to indicate that f[arg_1, arg_2, ...] should be considered a numeric quantity whenever all the arg_i are ...

NumberQ (Built-in Mathematica Symbol)

NumberQ[expr] gives True if expr is a number, and False otherwise.

Negative (Built-in Mathematica Symbol)

Negative[x] gives True if x is a negative number.

NonNegative (Built-in Mathematica Symbol)

NonNegative[x] gives True if x is a non-negative number.

NonPositive (Built-in Mathematica Symbol)

NonPositive[x] gives True if x is a non-positive number.

Positive (Built-in Mathematica Symbol)

Positive[x] gives True if x is a positive number.

ValueQ (Built-in Mathematica Symbol)

ValueQ[expr] gives True if a value has been defined for expr, and gives False otherwise.