Identify Files Already Parsed by Logstash

How do I know if a file is already parsed by logstash? I want to create a setup such that once a file is copied into a folder I will parsed it and move it to another location for archiving

There's no really good way of doing this, but you could inspect the sincedb files to get a sense of Logstash's progress. You can correlate the inode numbers in those files with the inodes of the files that you want to move out of the way. See the file input documentation for more on sincedb.