When you are looking at another person’s library that they have allowed for sharing, it tells you whether or not that account is in use.
The primary account holder can always launch a game any time, and like @Fraggles has mentioned, the person benefiting from the sharing is given a timed warning to either buy the game, quit the game, or I do believe the game force quits.
Shared games cannot be played in offline mode, for obvious reasons. However, you can time it properly and have the primary account holder restart in offline mode, to be able to play a game offline, so the sharee can also play if there’s any potential conflict. This is of course, if you are sharing with one other person.
Just keep in mind that setting it up might be slightly tricky, because if your accounts are locked behind the Steam authenticator, which I really hope most accounts are, then, you will have to deal with the signing in aspect of things. The person (primary account) needs to have their account signed into the person’s computer where the sharing will take place.
The 3 times I have done this, I have had both accounts make sure to change into some random/quick-n-easy password, shared the information via real time through these forums PMs, signed into each other’s accounts, allowed for sharing, allowed for that computer, you need to indicate sharing with individual and also their computer. Then sign out, each other sign into your own accounts to make sure that the sharing has been completed properly on both ends, then change your passwords again.