Depends on who makes it. I've seen some Chinese bars and plates that differ greatly. My York plates are quite accurate. They have these big swirls on the back that were milled by what appear to be giant Fly Cutters. I haven't seen any on the Chinese stuff.
Anytime you want to know the weight of something just stand on a scale with it and then subtract your weight from the total. You can use the same principle to measure weight carriages on machines and the like. If it's exactly vertical it is true weight. If it's on an angle then you must adjust by the percentage of the angle from vertical. i.e. A leg press loaded with 400 lbs including the carriage really only exerts 200 lbs of pressure if your legs are pushing up at a 45 degree angle. My leg press sits on a 45 degree angle, but the foot plate is at 70 degrees from my butt. So I have to figure on 70% of 90 degrees. Another way to check it is to just put the scale between your feet and the foot plate and then look at the scale after lift off.
Don't be fooled by the numbers game. Free weight movements are correct. A thousand pound leg press, though impressive, may really only be 500 pounds.