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From the rosnode ping --help output (also on the wiki page of rosnode (wiki/rosnode: ping)):

Usage: rosnode ping [options] <node>

Options:
  -h, --help  show this help message and exit
  -a, --all   ping all nodes
  -c COUNT    number of pings to send. Not available with --all

So this should do what you ask:

rosnode ping -c 1 <name of node>

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