@function my-content-exists() {
  @return content-exists(); // must be called within a mixin
}

@mixin uses-content-exists-indirectly {
  .test {
    error: my-content-exists();
  }
  @if false {
    @content;
  }
}

@include uses-content-exists-indirectly {
  .content { exists: yep; }
}