Probably the easiest way to recognize that a server is unresponsive is when 142 Destination Unreachable (Address unreachable) is displayed. Notice in this example that the first packet is an ping to IPv6 target fd00::8, and the packet details shows no response found. Notice also the protocol is ICMPv6.
It could simply be the case that IPv6 address fd00::8 is not assigned to any host in the network.
On the other hand, in this example, there is a successful ping to fd00::8.