Well! You are not very clever, are you? You don’t ask directly. There are many ways of collecting DNA samples from unwilling donors. Discarded trash is one. Cigarette butts. Use your imagination.
OK, you slip up on an African tossing some trash and he’ll have a bodyguard bloody you up pretty bad. Your ordinary African lives in such an impoverished state that there is no trash in their lives ~ only the rich have trash.