Checking that Pg is running from a shell script

What's the best way to ensure that Pg is running from a shell script?
 I'm crafting a few RPMs for my projects, one of which will attempt to
load data into a database, but I want to check that it's running first
before I try this.

I'm a shell scripting newbie so apologies if the answer is really
straightforward.  I don't know if UNIX processes leave a standard trail
so that I could just look up if something is listening on 5432, for
example.

Thanks,
Fran