Aside from the symptoms that were already listed, I think another sign is a difference in the way your pee smells.
I remember wondering the same thing as you, knowing that my breath may or may not smell and taste bad. And on the 3rd day I thought, when will I know if I'm in ketosis? The next day, I woke up in the middle of the night with a metallic-y taste in my mouth. Blegh. But you don't really notice that for long.
Then you just do what SusieB does.
I'd recommend a mouthwish that is alcohol free (like Biotene), so it doesn't end up inadvertently drying your mouth out, which is also a cause for bad breath.