Recently there has been a ton of posts asking if they should overline a rod, or what line for this rod. I almost always reply that it depends on the rod. Not all makers mark their rods as what they really are. In fact it is a rare rod that is what it is claimed to be. Given that fact, I generally tell them to test the rod and see what it really is. It is not hard to do and there are a lot of rods that have already been tested and the results published on the web.
Here is the Common Cents System method of telling what weight your rod really is: The Common Cents System
A chart that makes more sense than the one they provide and does larger rods: Full ERN chart
OK, now I am going to give you the fast easy directions to finding out what weight your rod really is.
1. Trap your rod by the handle parallel to the floor with the tip of the rod more than 1/3 the rod's length, higher than the floor. For example, you need to have the rod parallel to the floor and more than 3' off the floor for a 9' rod. I'd give it a foot to spare.
2. Take a small paper clip and unbend one end so it makes an 'S' shape. Find a small plastic bag.
3. Hang the plastic bag from the rod tip using the 'S' shaped clip, and then measure the distance from the tip to the floor and be precise.
4. Put pennies that are shiney and newer than 1996 into the bag till you have bent the rod tip to 1/3 the length of the rod from the measurement you got in step 3.
5. So for an easy example, lets say in step 3 you had a 9' rod and got a measurement of exactly 4.0 feet from the tip of the rod to the floor. Then in step 4 you got enough pennies in the bag to get a measurement of precisely 1.0 feet from the rod tip to the floor. Take the pennies and count them. For this example lets say you are testing an Orvis 'Zero Gravity Midflex' 5 wt., 9' rod. The number of pennies you count will be 53. This gives you an ERN, or actual rod weight of 6.5.
I would avoid the new nonstandard pennies, as I am not sure they are using the same slug to stamp them as the old style pennies. If they do any penny newer than 1996 is good as long as it is still shiney. This works because there is a known weight for these pennies.
There is a second way that is even easier, but does not work for all rods. Look it up for previously tested results. Here is a chart with a lot of results. If you look at our example rod you see a guy named Magnus has already done the work and calls it a 6.5 wt. rod. Sexyloops Rod Database
There was another site that had a great number of rods listed but they must not have paid to keep the site up and it has vanished.
Now in the case of the rod we used as an example, if you 'overline' it with a 6 wt. line, you are in reallity still underlined by a half wieght, provided the line maker didn't fib about that also.
Hopefully this helps with some of the questions.
Addition; If you use 1982-1996 CN pennies with no corrosion, it is a 1:1 swap. Both have the same weight.