The easiest way to test your website is to run python manage.py test localcrawler which will crawl your whole website starting at / including css, javascript and media files.
The test will fail if any link is broken.
If you would like a more detailed output, you can run python manage.py localcrawler which will also crawl your whole website starting at / including css, javascript and media files and print the result to your console at the end of the crawling.