lastlog(8) will report the most recent information from the
/var/log/lastlog facility, if you have
aulastlog(8) will make a similar report, but from the audit logs in
/var/log/audit/audit.log. (Recommended, as
auditd(8) records are harder to tamper with than
ausearch -c sshd will search your audit logs for reports from the
last(8) will search through
/var/log/wtmp for the most recent logins.
lastb(8) will show
bad login attempts.
/root/.bash_history might contain some details, assuming the goober who fiddled with your system was incompetent enough to not remove it before logging out.
Make sure you check
~/.ssh/authorized_keys files for all users on the system, check
crontabs to make sure no new ports are scheduled to be opened at some point in the future, etc.
Note that all logs stored on the local machine are suspect; the only logs you can realistically trust are forwarded to another machine that wasn't compromised. Perhaps it would be worth investigating centralized log handling via
auditd(8) remote machine handling.
You can also try:
grep sshd /var/log/audit/audit.log
last | grep [username]
last | head