If I have the IP address of a computer running Linux is it possible to see which user accounts exist on that computer, without having to ssh into it first?
Nowadays, on a correctly-configured system there shouldn't be any way of determining this.
On a badly-configured system, you might see different behaviour when attempting to log in depending on whether the user you're trying to log in as exists or not. (Hopefully not in the messages returned, but perhaps in the timing of the responses.)
In the past,
finger would have told you:
But those days are long gone...