[NumLock]

Есть готовая функция Sign:

Indicates whether a numeric value is positive, negative, or zero.

Unit

Math

Category

Arithmetic routines

Delphi syntax:

type TValueSign = -1..1;
function Sign(const AValue: Double): TValueSign; overload;
function Sign(const AValue: Integer): TValueSign; overload;
function Sign(const AValue: Int64): TValueSign; overload;