How to display the current DNS servers from the command line under OS X

To get the settings:

networksetup -getdnsservers Airport

To change them:

networksetup -setdnsservers Airport xx.xx.xx.xx

These settings are non-persistent, so you could clear your manual settings, let the public wi-fi DNS do its thing, then use the -setdnsservers to set it to what you want until next time.

You could use mdfind, which uses the Spotlight database. More information on this man page.