Index Usage Statistics

I am looking for a way to determine when an index has become so fragmented that it should be re-built. An application I am developing continually deletes and inserts rows into a large table. I don't want the program to recreate the index every time a large number of deletes and inserts have occurred, because of the time it takes to rebuild the index. I would like to have the program query the status of the index, and display a message when the index needs to be re-built. Thanks in advance, Drew