gardenguy :
How can I tell if my motherboard will handle a sata3 hard drive at its 6gb/s rate?
Well you would need to know "who your motherboard is made by" and find the "model number" on it.
Once you have those 2 things, you can go to the makers web site and type in the model number there and on there product page it should state if it's sata 2 or sata 3.
Or as most motherboards do (
but not all), there will be different sata connector colors on the Motherboard that will mean different speeds. although unlike USB 3.0 where all those ports are blue, the different sata ports color will vary on the maker.
Although i will state that if your just using a HDD and not a SSD, you wont really see a difference between sata 2 and 3 as HDD can not operate fast enough to use such bandwidth.