Firstly, you get unexplained weight loss. This is because you can't metabolise sugar any more, and the only energy your body can process is fat that's already there. When that's gone, things get worse.
In my case, I suffered severe dehydration. It didn't matter how much water I drank, it would just come out the other end half an hour later without giving me any of its watery goodness. I would frequently wake up at night with my tongue swollen and cracked from dehydration and needing to go to the loo. I was also tired all the time and developed a rash on my back, possibly due to a build up of ketones turning my blood into acid.
If you're worried, go see a doctor for a blood test that takes 10 seconds. Or if you know anyone who is diabetic, they might let you use their testing kit.