How to tell when postmaster is ready

I'm writing a small test harness. I have two threads. One that starts
the postmaster and another that does all the testing and finally stops
the postmaster with a pg_ctl stop. At present, the second thread starts
with a sleep sufficient to ensure that the postmaster is running. Is
there a proper way to test when the postmaster is ready to receive commands?

Kind regards,

Thomas Hallgren