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

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

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 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 ...

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

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

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

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

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

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