You definitely want to Option Boot the laptop to make sure there is no firmware lock:
https://support.apple.com/en-us/HT204455
You also want to make sure the laptop is not being managed by a school or business or still associated with the former owner's AppleID where they could just lock and erase the laptop at anytime. This is hard to assess even if you have lots of time. Usually if you perform a clean install you will notice if a school or business are still managing the laptop as you may see the notice or you may see unusual apps installed on the laptop after a clean install. However, this won't tell you if the previous owner's AppleID is still in control until you attempt to associate your AppleID with that device which you do not want to do until you perform a clean install of macOS. This isn't something you can do or tell in a parking lot.
Keep in mind that if a Mac ever has a firmware lock enabled for any reason (even if it just spontaneously happens -- I've seen that occur one time with a co-worker), then you will never be able to remove that firmware lock unless you know the firmware password since you will not have valid proof of purchase which Apple will accept to remove the firmware lock.
I think it is extremely dangerous to buy a pre-owned Mac from an unknown source for this very reason since Apple does not provide their users with a method to properly change ownership of a Mac so that you could have a firmware lock removed if necessary.
