So, if I am on my home network, I can see other network devices, such as my girlfriends PC. If she has a share enabled, I should be able to double click on her computer to reveal the share. Then if I try to open the share, it states I need credentials.
At this point, I would put as you say:
HerPC\Username
Password
As for discovering inside a domain for anything external. Not 100% sure, never tried it. I doubt it will be as simple as above for a home network.