Depends on the people, but typically all the places you would look for people between census records and creating a vital record. Unless you know a bit about them, it can be difficult since they could have changed names without going through the courts. In any case, if you have some idea of where they were, tax records, property records, probate records if they died, city directories, court records (we are a litigious country) and other records that record any types of transactions.