The simplest way to to check running processes:
ps auxwww | grep postgres
And look for a command that looks something like this (your version may not be 8.3):
/Library/PostgreSQL/8.3/bin/postgres -D /Library/PostgreSQL/8.3/data
To start the server, execute something like this:
/Library/PostgreSQL/8.3/bin/pg_ctl start -D /Library/PostgreSQL/8.3/data -l postgres.log