(define-syntax if-has-placeholder?
   (syntax-rules (_)
     ((x T F) F)
     ((x  T F _ arg ...) T)
     ((x T F arg0 arg ...) (x T F arg ...))))

(if-has-placeholder? 1 2 _ 3) => 1
(if-has-placeholder? 1 2 3 4 _ 5) =>
  Error: unbound variable: x