If your posts are less than their named dimension (I.E. less than 4x4, 4x6, or 6x6) you have a Nominal post. If they are equal to or greater than their named dimensions, you have an Actual or True post.
The easiest way to be certain is to measure!
If you've recently purchased 4 in. x 4 in. wood from a home supply store, you've most likely got a piece of wood that is actually 3.5 in. x 3.5 in. which is a nominal measurement. Be sure to measure if you have any doubt!
When you see the word 'Nominal' next to lumber sizes, it means that the actual dimensions of the wood are smaller than what the sales name implies.
The reasons for this are based on how wood has been shaped through the years. Before precise machinery, a 4 in. x 4 in. was 'green wood', or wood that was rough and unfinished. After drying and planing, the wood would be approximately 3.5 in. x 3.5 in. Although this can often cause consumers considerable frustration, these standards are a result of lumber industry agreements and create a reliable way to ensure the correct sizes of lumber are obtainable.
As the name suggests, an actual measurement is the actual dimensions of the lumber. So, if you purchase a 4 in. x 4 in. (actual), you are getting a piece of lumber that is genuinely 4 in x 4 in.