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.
4. If your domain is on an Enterprise plan, check the additional firewall analytics listed in the Firewall app or your request/bandwidth analytics in the Analytics app (all plan types) for clues such as spikes in traffic.
Related: Mitigating a DDoS attack