I believe it's up to the programmer to implement this. Here's some code from the zig github[1] page that illustrates how it might look once zig is stable (this is from an issue).

  pub fn write(file: *File, bytes: []const u8) usize {
    // Note that this is an if with comptime-known condition.
    if (std.event.loop.instance) |event_loop| {
        // non blocking version
    } else {
        // blocking call
      }
  }

So I guess functions that don't support async could just do something like:

  if (!std.event.loop.instance) @compileError("X function only supports async");

[1]: https://github.com/ziglang/zig/issues/1778