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 -v
Or, if you don't have curl installed, you can use wget:
$ wget -q -O - http://drupal.org/files/[patch-name].patch | git apply -v
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.