What are the tell-tale signs that an online service is currently under a DDoS attack?

Key signs of an online DDoS attack include an unexplained surge in traffic from a large number of IP addresses, especially from unusual geographic locations; sudden and dramatic slowing of network performance; complete unavailability of a website or online service; issues connecting to an internet service; and a significant increase in requests to a single endpoint or resource, often accompanied by server error messages.