For many drugs if you knew the route of administration (orally for example) and the time, you could get a pretty good feeling of the dose.

The pharmacokinetics of drugs give you a curve or a line of some kind and it is possible to back calculate.

There are certain caveats. For some things , the original drug is labile and the metabolite in stable, so you would just measure that. But this is a technical thing, like measuring an oxidized version rather than not. Some things are in bound and free forms and it is technically difficult to get both. Some things have endogenous versions that make it hard to tell what you ate vs what you had (erythropoietin , testosterone) because they are indistinguishable. Some things have such a long or short half life that it becomes complicated. Some things that are stored in fat are tricky to figure out doses of. And all sorts of other little quirks. But , a qualified yes is really the answer.