Just like @LoyceV said, remove the HDD before giving the device to someone else. If they ask why the HDD is missing, you could say you need the data for urgent work and ask them to use their own HDD/USB drive for testing.
I am not sure many laptop allows for an easy HDD removal, even more without breaking warrant (Mac User anyone?).
Then you're either forced to,
1. Learn removing HDD from your laptop and buy necessary tool (e.g. screwdriver for screw type used on your laptop).
2. Perform secure disk wipe using bootable OS on flash drive.
3. Fully trust the technician.
Basically, when to give your laptop to an external technician, you are trusting him not to rip you off.
True, but it's less likely they will do something obvious (e.g. charging very high "repair" cost) rather than cloning HDD which hardly could be detected.