How can I tell if my Macbook Pro has had its SSD replaced or not?
If you happened to have written down your hard drives serial number before handing it in for repairs, you could detect replacement by comparing the serial numbers.

You probably haven't written down the serial number. Then you could try looking at the drive physically to determine the production year, if written on it. If that is newer than the year your MacBook Pro was bought - it has been replaced.

You do not mention which model of MacBook Pro you have, but it must be one of the older models to have a hard drive. It could be possible to check if the disk you have is not one of the models that Apple installed from factory on your specific MacBook Pro model.

Also you could consider asking your IT guy how he knows the hard drive was replaced. Perhaps he has paper work to document it.