I know that things like sodium, cholesterol, and sugar levels can be found in blood (at least I think so.) But what about things like fruits and veggies? Can that be detected through bloodwork? Or would you only be able to see the chemicals/molecules that it has broken down into?
From the results of usual blood tests, such as glucose, protein, cholesterol, minerals and vitamins, you can't see which foods the person was eating, because these substances are not specific for a particular food and some of them are produced in your body.
There are specific IgG antibodies in the blood that form complexes with certain food particles (antigens), from which you could see which foods the person was eating.
Type III hypersensitivity reactions, associated with IgG antibody-mediated response, are a body's normal reactions to food antigens, which are usually absorbed into the bloodstream in small quantities. Healthy people produce and maintain high IgG antibody titres against food antigens. After a meal, there are both antibodies and complexes of food antigens bound to specific IgGs circulating in the serum.
From such tests, you could see which foods the person was eating, let's say within last few months, but you could not tell when exactly.