貌似没有很优美的做法，只能把%Path%按分号分割，然后一段一段地比较。

:InPath [In]Path [Out]0/1
SETLOCAL
SET LocalPathCopy=%PATH%
SET Result=0
:WHILE
IF "%LocalPathCopy" == "" GOTO WEND
FOR /F "delims=;" %%I IN ("%LocalPathCopy%") DO (
  IF /I "%%~I" == "%~1" (
    SET Result=1
    GOTO WEND
  )
)
FOR /F "delims=; tokens=1,*" %%I IN ("%LocalPathCopy%") DO (
  SET LocalPathCopy=%%~J
)
GOTO WHILE
 
:WEND
 
:InPathRet
ENDLOCAL & SET %2=%Result% & GOTO :EOF

使用方法：

SET x=C:\Windows
Call :InPath %x% Result
IF %Result% == 0 SET Path=%Path%;%x%