Given the context of wanting to reboot a Linux system, I would take a multi-pronged approach.
First, disable future logins by creating an /etc/nologin file. You could leave it blank or enter informative text in there, such as:
"Logins to this system have been temporarily disabled in preparation for a server reboot, scheduled for (time and date). Please try again after (expected end time)."
Don't forget to remove /etc/nologin when you're done!
Additionally, since a reboot will clear all processes, whether they're interactive or not, I would use ps to look for processes owned by users. This will take some manual investigation to determine whether the processes are worth keeping or not, but should narrow the field some. I've hard-coded 1000 here as the value of UID_MIN from /etc/login.defs as the cutoff for "system" vs "user" UIDs. If any of your users have UIDs below 1000, you'll need to adjust that number.
ps -eo pid,uid,args | awk '$2 >= 1000'
Of course, you could adjust the ps columns to taste, perhaps to add the translated username and process start time (ps -eo pid,uid,user,start,args) or others -- just be careful to keep the ps UID and awk field in sync with each other.
To get the list of unique user names, use:
ps -eo user,uid | awk 'NR>1 && $2 >= 1000 && ++seen[$2]==1{print $1}'
