heroku DATABASE_URL

$ heroku config --app APP_NAME 

(The Heroku toolbelt has been updated since the question was asked; the command now shows the entire DATABASE_URL.)

$ heroku pg:credentials:url YOUR_DATABASE_NAME

Gives you all the informations needed (url, port, dbname, user, password):

Connection information for default credential.
Connection info string:
   "dbname=xxxxxxxxxxxxxx host=yyyyyyyyyyyyyyyyyyyyyyyyyyy.amazonaws.com port=5432 user=zzzzzzzzzzzzzz password=************* sslmode=require"
Connection URL:
   postgres://zzzzzzzzzzzzzz:*************@yyyyyyyyyyyyyyyyyyyyyyyyyyy.amazonaws.com:5432/xxxxxxxxxxxxxx