The most common tip-off that you are either under attack or functioning as an amplifier is sluggish network or server performance. Other indicators are:
High router CPU utilization.
Asymmetric traffic patterns. Targets will see excessive inbound and minimal outbound traffic. Amplifiers will see excessive outbound with average inbound traffic levels
Unexplained system crashes.
IP traffic from multiple hosts with the same IP prefix.
IP traffic with destination addresses ending with .255 or .0