Monitor if users accessed my Mac when sharing was open to all?
The file sharing protocol AFP used by OS X does not log access per default. While (at least for OS X Server) reporting to a log file can be activated before starting file sharing, it is quite a "chatty" log process (meaning it generates huge, meaningless log files easily).

To put it simple: No, there's no way to find out who accesses your Mac beside yourself after file sharing ended.