That doesn't make sense. The server would wait to process anything until after you received the signal. If it doesn't arrive within a reasonable period of time that tells you something, the same as right now.

If you mean that you can infer client side tampering with the page contents you could still do that - permit typing but don't permit the submit action on the client. The user presses enter but nothing happens until the check is complete. There you go, now you can tell if the page was tampered with (not that it makes much difference tbh).