No they can not tell from a pap, as far as I know. Most doctors assume that is the reason as HPV is the cause of 80-90% of abnormal paps and cervical cancer. However there can be other causes. Smoking, birth control, genetics, diet, lacking certain vitamins. There is a blood test that will tell if you have the HPV virus. More then generally doctors do not run this though because statistics say HPV is the general cause. If it is HPV no it will not make all of your paps abnormal if it is treated. If the doctor does not think it is severe enough to treat you probably have an infection or mild dysplasia which they will monitor closely to make sure your body heals itself and it doesn't progress.
If you have had the thin prep pap and it was abnormal, an hpv test is usually done. No, it won't always cause an abnormal pap, especially if the results were mild dysplasia. Chances are it would clear on it's own and revert back to a normal pap. But it may come back later on and that's why it's important to keep up with getting your paps. Your doctor may recommend paps every 3 or 6 months for a while, or recommend treatment, depending on the results. As far as I know there is not a blood test for hpv as the previous poster stated. It is just done if your pap is abnormal. If your pap is normal then there is no need to test for the hpv virus.