There are a few easy to spot clues that you're having a DDoS attack against your site. They are:
1. Your site’s responsiveness will be very slow to respond to requests and/ or be totally unresponsive to requests.
2. If you check your server stats, you will also see a huge increase in visitors or bandwidth requests in a short period of time being directed against the site.
3. If you look at the server logs for your site, you will also see a lot of strange requests to your site that don’t match normal visitor behavior.
Related: Mitigating a DDoS attack