How can I tell if someone copied files off of my Internal HDD? [duplicate]

There is no way to tell if your drive was accessed reliably. There may be indications that the drive was accessed, but the absence of these indicators does not mean that the drive was not accessed.

If a drive is mounted read-only or simply cloned, no information gets written back to the disk. Thus, you won't be able to find any differences (apart from SMART information, but you would need to know the exact characteristics before the drive was given to someone else).

Additionally, since you apparently already accessed the drive in read-write mode, you cannot tell whether timestamps etc. were modified by the other person: it might have been your system (without you realizing it). So unless you find new data of which you can tell with 100% certainty that it was not coming from your system, you're not going to be able to tell whether the other person has accessed your drive.