I've had a look through the documentation, and I'd have to say I couldn't find any way to detect and display the current vi-mode. I do hope you have a pleasant surprise and someone comes up with a solution, but I certainly can't see a way to do what you ask.
I think there is a way to do this, but it is massive overkill for a tiny annoyance, and I don't think that's what you had in mind when you posed this question. If this is true, stop reading now and enjoy your life.
OTOH, if this really is your pet peeve, and it's driving you crazy, and you really absolutely desperately wanna gonna smack this problem, here's my idea of how to go about it:
- Get a copy of Advanced Programming in the UNIX(R) Environment by W. Richard Stevens.
- Read the chapters on Streams and Terminal I/O.
- Download the source code to the aforementioned book, which includes an example of a stream that can be layered onto a terminal.
- Implement your handling for the Esc key combination (or both mode change keypresses), and indicate it via a bell or background manipulation of the current line.
In brief, Unix implements terminal I/O as a full-duplex I/O stream between the device driver and the user process, into which modules can be inserted. It is organized as a stack, so you can layer as many streams as you want. The sum of the injected streams creates your terminal I/O behaviour.
When a character is entered, the first module gets to inspect it and pass it on (if it wants to). When the response arrives in the opposite direction, it again gets a chance to inspect and pass it on. This is how a Ctrl-C gets handled at a higher level to a normal character key.
You can create a binary implementing a stream module, that when invoked runs silently in the foreground, inspecting and passing on all keystrokes, performing your preferred action on the keystrokes you care about. For all intents and purposes, it'll appear as if you're working on the shell. Oooh. You can invoke this binary on the last (or first, or any) line of your .tcshrc and you wouldn't even know it's there.
There's a good primer here, but I couldn't find much on this topic, probably because it's past its prime.