Testing a patch file is usually a two-step process. First you download the patch file from the source, and then you run a separate command to apply it.
You can save time and typing by running the two commands on one line:
$ curl http://drupal.org/files/[patch-name].patch | git apply
Or, if you don't have curl installed, you can use wget:
$ wget -q -O - http://drupal.org/files/[patch-name].patch | git apply
These commands need to be run within the root of your Git repository (i.e. where the .git directory is).
These snippets were taken from Applying Patches with Git on Drupal.org.