‘Marked man’ is most commonly found in the phrases ‘to be a marked man’ or ‘to live like a marked man’, and is used to refer to a boy or man who is being observed by somebody, not necessarily enemies, because he has done something wrong and may continue doing it.

My brother is now a marked man; my father suspects that he’s stealing money from him.’ (i.e. my father suspects that my brother is stealing money, so my father is observing him so that he can catch my brother stealing, or not to give him any chance of stealing)

‘To live like a marked man’ is to be living in fear of danger to one’s life, as in ‘Americans staying abroad are living like marked men because of the threats from Al-Quaeda and other terrorist groups.’ (i.e. some terrorist groups are targeting Americans and are killing them wherever they are found; therefore, the Americans visiting or working in other countries are constantly under the fear of being killed or harmed).