The command last can be used to figure out who last logged in locally! Also, lastlog can used to see when the user logged in over the interwebs last.
last command
Show who is still logged in:
last | grep -i still
Example show the 4 last times logged in:
last -n 4 <username>
lastlog command
Show all users who last logged in:
lastlog | grep -v '\*\*Never'
When was the last time the logged in:
lastlog -u <username>