monitoring postgres

I subscribe to a server monitoring service that notifies me if any of my
public servers stop responding to periodic queries.

It has predefined functions for monitoring standard web facing services
such as ftp, telnet, http, https etc.  They also offer a custom function
for other services, which is what I need to use to monitor my postgres
servers.

They do a challenge and response type query where they send a specific
message on UDP or TCP port of my choosing and if they don't get the
response that I specify then they send me a page.

The problem is that I block traffic to my Postgres servers at the
Postgres level using a list of acceptable hosts that can connect to the
server.  I don't want to add their hosts to my server's allow list.

I'm not blocking them at the firewall, so they can see the server on
that port, but can anyone suggest a text string and expected response
that I can use to know that the server is OK?

--
Matthew Nuzum
www.bearfruit.org
cobalt@bearfruit.org