The body will always work in our favour - it will neutralise acidic blood by depositing toxins into our fat cells (in fact the opposite should be happening - our cells should be depositing THEIR toxicity into the blood for elimination via the liver). Toxic cells become oxygen deficient and need to change to survive. Cancer cells are able to function anaerobically, (using sugar to metabolise), meaning that they can function in this oxygen depleted environment. In addition, calcium is pulled from the bones in an effort to alkalise the blood. This can lead to low bone density and osteoporosis.
Our modern diet is extremely acidic; meat, dairy, sugar, grains, caffeine, alcohol, processed foods and medications are some of the worst offenders. At the other end of the scale are the extremely beneficial foods which are highly alkaline: potassium rich greens, most organic fruits and vegetables. But it's not so clearcut. Even some foods we would perceive as healthy are acidic. The key, as always, is in balance, and we should aim for an 80:20 ratio of alkaline:acidic foods.
It's easy to check and monitor your PH with PH strips using either urine or saliva - ideally they will match. pH varies at different times of the day, depending on what you've eaten, whether you've exercised, and stress levels (!). The best way to test is several times a day over a 5 day period. The first urination of the day is not representative of your normal pH because it contains all of the acids filtered out through your kidneys during the night, so test your second urination of the day, then again before lunch, and then before dinner. It's important to check pH before meals because it will vary considerably depending on what you've eaten or drunk (try experimenting with green juices which are incredibly alkalising, and will change your PH almost immediately - a good reason to work up to 3 or more juices a day).