GnuPG, a common free software implementation of OpenPGP has versions and graphical frontends for both Windows and Mac OS X. This also make it possible to check the cryptographic signature with those operating systems:
You will find on either of those websites detailed documentation on how to install and use them.
After installing Gpg4win, download Tails signing key:
Then, download the cryptographic signature corresponding to the ISO image you want to verify:
If you see the following warning:
Not enough information to check the signature validity. Signed on ... by email@example.com (Key ID: 0x58ACD84F The validity of the signature cannot be verified.
Then the ISO image is still correct, and valid according to the Tails signing key that you downloaded. This warning is related to the trust that you put in the Tails signing key. See, Trusting Tails signing key. To remove this warning you would have to personally sign the Tails signing key with your own key.
After installing GPGTools, you should be able to follow the instruction for Linux with the command line. To open the command line, navigate to your Applications folder, open Utilities, and double click on Terminal.