Using lsof as suggested is a good way to know if something is accessing it at that moment, and is a good solution but you can go with something more sophisticated that will allow you to monitor what's happening during the time.
Auditd is a service included in CentOS that monitors different kinds of actions, so you could add a rule to monitor all access to /old
auditctl -w /old -p war -k old-access # old-access is an user defined string
Then when you want to see if something has accessed the directory you can use ausearch to look into all the events using the define key "old-access"
ausearch -k old-access
If you want to make this change permanent across reboots add "-w /old -p war -k old-access" to /etc/audit.rules (and remember to apply the change with "service auditd restart".
If you want to test it just run "ls /old" and after that "ausearch -k old-access" will show you all the events.
You may have some processes (mlocate or some integrity check like aide) checking the filesystem that may be accessing those files, so you will have to look at the results, but at least you will know if that directory has been used after 1 day, 1 week or whatever you would like to wait to make sure that the content isn't important.