You should first analyze your
VPS Hosting control panel, HyperVM or Virtuozzo, and find your bandwidth (traffic) graphs. If you notice very high (and constant) graphs, that means you are probably under a DDoS attack.
But, if your VPS server is hosted at Copahost, there are some firewalls before your VPS node, whenever we find a DDoS we will email you as a customer and let you know what's going on, and what ip(s) should we block then.