script to monitor for new files in a shared folder (windows host, linux guest)

You need something that polls for file changes because if a file is modified on the Windows side, the Linux kernel is not going to know about it. There are a few existing applications that can help with that, such as Guard: http://guardgem.org/

Depending on your exact needs, you could just watch the file listing (adjusting n seconds to whatever is suitable):

watch --differences -n 10 ls -l </path/to/shared/dir>