You can use the who command to see if particular users are logged into the server. The who command will display information about users who are currently logged into the system, including their username, terminal, and the time they logged in. For example:
who
user1 tty1 2021-02-11 08:30
user2 tty2 2021-02-11 09:45
You can also use the grep command to filter the output of the who command and display information only for a specific user. For example:
who | grep user1
user1 tty1 2021-02-11 08:30
This will display information only for the user user1.