Currently, there doesn't seem to be any visible indication that a pet is juggernauted other than a small icon in the pet window of the user. It would be nice to have the pet get bigger, but even an animation like the one for charmed minstrel pets would be useful. I think this would help people who are coming up against a juggernauted pet realize what's going on and react better, but it would also help those who are using juggernaut so that they get a better feel for how long it lasts and when they might need to start being more careful with their pet again.