Ok, fair enough. Guess I was kinda vague. Thanks Ekbustad and jar nick for the CSA definition. Yes it is the Confederate States of America.
The best way is to compare the stamp with a known example. Easily you could look at other CSA #4s (thats the Scott's number) on
ebay or elsewhere on line.
But I'll compare it to my own copy of a CSA #4 Stone 2. How do you know YOURS is authentic, you'll ask. I'm no expert by any means, but a close examination of images online of known examples tells me this its most likely real. Also when I first acquired this stamp (on the left in the image below), I sent a picture of it to Dr John Kimbrough, a highly respected CSA expert in Texas, and Dr Kimbrough very kindly and generously looked at my stamp and confirmed it was legitimate. Not a true 'expertizing' but good enough for me.
So look at the two stamps in the image below, authentic on the left,
ebay fake on the right, and I expect, and hope it should be very clear that the sample on the right is a very crudely drawn replica. I could go over a number of points, the different serifs on the S in CENTS, the broader shapes of the swirls in the bottom corners, or the difference in the shapes and curve of the word POSTAGE.

But mainly, look at the portrait of Jefferson Davis. That's NOT the same guy! The drawings are completely different, the rendering on the portrait on the fake much cruder and poorly executed.
And if you can't take my word for it, check out the website of another expert Trish Kaufmann. On her page 'Crude Forgeries', this stamp can be seen, printed in green, on the bottom row, second from left.
http://www.trishkaufmann.com/images...rude.jpg.jpg