We know that many cancers are a result of this. For example smoking related cancers of the lungs and oropharynx, alcohol related cancers to the oropharynx, gast

We know that many cancers are a result of this. For example smoking related cancers of the lungs and oropharynx, alcohol related cancers to the oropharynx, gastrointestincal cancers from H. Pylori, Adenocarcinomas of the oesophagus from stomach acid due to GORD, some squamous cell carcinomas of the skin. Yes it is detectable, to an extent. But inflammatory markers change on an hour to hour basis, so a one-off test is not useful. And There is no benefit to testing on an extremely regular basis. Chronic high-level inflammation is easier to detect as there are certain proteins and blood markers that only change over the longer term however these are signs of chronic disease or even cancer itself and thus not really useful as a prevention. The cellular dysplasia that occurs is a better sign that some pathological process is underway and we utilise this information, collected through biopsy of skin lesions, examination of pap smear cells and other biopsy specimens to work out if we can do some procedure to reverse a potentially cancerous lesion. But there is no way to work out some global low level inflammation as the insult that causes many of these cancers occurs locally, and you really need to look at the tissues in the areas of the insult (as we already do) to see how far along it is; even if you could perform a blood test that would tell you that 'there is some low level chronic inflammatory process occuring that may lead to cancer' you would need to be able to localise that inflammation in order to work out how to reverse it