how spooling directory source identifies the complete file

Hi,

I am planning to use spooling directory to move logfiles in hdfs sink.

I like to know how flume identifies the file we are moving to spool
directory is complete file or partial its move still in progress.

if suppose a file is of large size and we started moving it to spooler
directory , how flume identifies that the complete file is transferred or
is still in progress.

Please help me out here.

Thanks,
saravana