Testing a String for a number that can only contain the ciphers 0 to 9 and optionally a prefixed - character can be done with the following subroutine.

:: Checks if the first parameter is a number, that is it only contains the
:: characters 0 to 9 and additionally the minus as the first character.
:: PARAM
:: 1 string to test for number
:: RETURN String "true" if string is a valid number, "false" otherwise
:IS_NUMBER
SETLOCAL
FOR /F "tokens=*" %%A IN ('SET /A %~1 2^>^&1') DO (
    IF "%~1" == "%%A" (
       SET RESULT=true
    ) ELSE (
       SET RESULT=false
    )
)
ENDLOCAL & SET RESULT=%RESULT%
GOTO :EOF

This tests the input string using the SET /A arithmetic extension.
However this test two limits I can think of:

• As SET /A echoes the error Missing operand. on invalid input, a test against that very string will return true.
• A number with leading zeros like 007 will not be recognized as a number.

If you cannot live with these limitations, you have to replace this relatively fast solution with two loops doing something like this:

FOR EACH CHARACTER (
    FOR %%A IN (0-9) (
        IF (%%A == CHAR) (
            GOTO :END_INNER
        )
    )
    GOTO:EOF & FAIL
    :END_INNER
)