How can I check a function return type in another function in vb script.

Um... OK, so that's some basic VBScripting there.  Simply...

Function A()

     Set obj = ----------something here-----
     A = Obj.Exists

End Function

And then, in function B

Function B()

     If A then
          Log.Message("Object exists")
     else
          Log.Message("Object does not exists")
     End If
End Function

You can try below scenario and check if it works for you

Function A()

     Set obj = ----------something here-----

    If(Obj.Exists) then

         Log.message("ok")

         strResult=TRUE

    Else

        log.message("not ok")

       strResult=FALSE

    End IF

   A=strResult

End Function

Function B()

  strResult=A()

   If strRESUlt=TRUE Then

      log.Message"True"

Else

     log.Message"False"

End Function