1 more thing to consider is that if you are useing a "loaner" from autozone etc then how can you make sure the torque wrench was set back to 0 when used last?
if not set back to 0 after each session this will throw out the settings and mean the torque wrench is useless!
This is not directed at anyone, just some general comments.
The answer is you don't and you also don't know if the tool you are buying is any better. Sure it comes from the factory but after you use it once, how do you know if it is correct. How many people get their torque wrenches calibrated? The torque recommendations are not that exact. If it is off by 10%, than it will not hurt anything. This is not brain surgery, this is a car. I would be willing to bet that after a car leaves the factory, 95% of the nuts and bolts replaced on a car are not torqued to the proper specs. While there are things on a car, mainly the engine, that need proper torque, the rest is not that important. I lived near a tire shop when I was growing up, and I never saw them use a torques wrench, in fact I have never seen any tire shop use a torque wrench to put on wheels. People need to relax a little bit. And don't give me that crap about the car will fall apart and it could hurt someone. Do you realize how much of a safety factor they use in something like that? Sorry for the rant and goodluck with your car repairs.