The query instant subcommand allows the querying of the Prometheus server directly via the command line based on the current time. For it to work, a Prometheus server URL must be supplied as an argument, as well as the query to execute, like so:

vagrant@prometheus:~$ promtool query instant 'http://prometheus:9090' 'up == 1'
up{instance="prometheus:9090", job="prometheus"} => 1 @[1550609854.042]
up{instance="prometheus:9100", job="node"} => 1 @[1550609854.042]