There's a helpful little comand called last that will show the historical logins of your Linux system's users.

Turns out, there's some additional flags you can add that will make it show a lot nicer output:

last -a -F -w -x

This has the advantage of showing if you have different ways of accesing the system, like mosh, this is called out as well.

Additionally, there's another command that's useful called lastlog that shows each user on your system and the last time that account has authenticated (if they have at all).

lastlog