The Netflix "USE method first 60 seconds":
uptime # load averages — system overall load
dmesg | tail # recent kernel messages, OOM kills
vmstat 1 # virtual memory, CPU breakdown
mpstat -P ALL 1 # per-CPU stats
pidstat 1 # per-process CPU
iostat -xz 1 # per-device I/O
free -m # memory in MB
sar -n DEV 1 # network throughput
sar -n TCP,ETCP 1 # TCP throughput and errors
top # interactive process view
You won't run all of these, but knowing them signals SRE maturity.