There are two mechanisms for capturing packets on the ASR9K, SPAN and an NP capture. The NP capture you have to know which NP counter you want to capture on, for drops. For SPAN check out https://supportforums.cisco.com/document/60596/asr9000xr-how-use-port-spanning-or-port-mirroring
Packet loss is a little more trivial. You need to look at the interface counters, the NP counters, fabric counters, etc to look for any loss. show controller np counters, show controller np ports, show controller fabric fia, show controller fabric crossbar, etc
Thanks,
Sam
