As far as authenticity outside of paperwork and stickers, knowing what kind of marks, machining, colors/finishes, and other features will help you identify authentic wheels.
--Also, you can look for areas where the paint or metal looks patchy or the surface is not quite the same for repairs. Nothing bad about a repair (a good shop can get it almost new), but it will tell you about the character of your seller if they aren't disclosing with you.
As far as size goes, usually authentic wheels have the diameter, width, and offset stamped/cast into them. It will usually look something like "17 x9 JJ," and offset is usually nearby in mm
If you have a relatively flat garage surface and
unmounted tires, rolling the bare wheel can tell you if there is a bend if the motion is irregular or wobbly. Otherwise, rolling a wheel can give you a rough idea if the wheel is straight by observing it.
As far as tires go, check the manufacture date. That will give you an idea of how old the tires are and if they're still good (outside of treadwear depth). Also look for sidewall bulges and potential patch/repairs.
Like @
Mr.ac said, if you don't have any experience with wheels or tires, a wheel/tire shop is probably your best bet on getting a 2nd opinion as far as structural integrity. Not all shops deal with Import/JDM wheels, so they might not know a real from a fake as far as brands.