Gum disease is usually first detected by seeing blood in your sink after brushing or flossing. If you bleed after you clean your teeth, floss more and see your dentist. Avoid sugary diets and sugar habits such as sipping on soft drinks. Also, it is a good idea not to share tooth pastes between family members as colds and other infections can be spread from one toothbrush to another from the tube of toothpaste.
To answer your second question, it can hurt a lot to have a tooth worked on without anesthesia. Again, it will depend on the size of the decay, the skill of the doctor, and other factors like your pain threshold and age. Older patients have smaller nerves and don't feel nearly as much as a younger patient who typically have huge nerves. You can always tell your dentist to try and go slow and if you start to feel pain, get numbed up.
Common causes are root exposure and clenching. Root exposure can happen naturally in life, but many times it is caused by aggressive brushing or excessive clenching. If your tooth brush looks 'bent and worn' after a short period of use, then you are brushing too hard and I recommend a Sonic brush. You can't brush too hard with one of those.
Also, you might ask your dentist to make you a night guard. Clenching leads to sensitivity. Ask yourself from time to time, 'are my teeth together right now?' If they are, you are clenching too much. Your teeth should never touch unless you are eating. Period. If you are clenching and you control that, your sensitivity might improve.
Good luck, Doctor Hampton