Hi! That actually depends on what kind of a lemon you expect it to be :)
I would first start with trying to boot it "as-is" and see what happens. Do not connect it to any network at this stage, as you don't know if it is full of malware or not. If it does not boot, there is no reason to panic (yet). Get a bootable USB with an OS that can run off removable media (e.g. Ubuntu) and try to boot off it. Once you boot, you can test graphics, sound, etc. If you booted from a USB media, you can also test network, as you trust this OS.
If everything is fine until now, you have a decision to make. Do you trust this OS enough to use embedded functions to clean it (won't remove most of malware), or do you want to install from scratch (will remove everything off the laptop). Some of the laptops may have a recovery partition that you can boot off and get the computer to an "out of factory" state. I wouldn't recommend this in most cases as the OS will be old and will require a lot of updates.
Let me know how you would like to continue and we will guide you through :)