The plugin should have information in it regarding the license in use. Look in the readme file (could be called README, readme.txt, README.TXT, readme.md, or some other permutation), the plugin’s PHP files (there will be one with a block that begins with:
/*
Plugin Name: xyz
.
.
.
*/
and that block will often contain the type of license in use. You may also find a License file — possibly LICENSE, license.txt, license.md, or some permutation.
The License section of the Codex page on Writing a Plugin states that
Most Plugins use the GPL2 license used by WordPress or a license compatible with the GPL2.
Read up on the GPL (GNU Public License) here. (Note that the current GNU page references GPL v3, not v2. I don’t know what differences might exist between v2 and v3.)
Markdown on Save Improved was forked from Markdown on Save, and includes this text in its code:
This software is forked from the original Markdown on Save plugin (c) Mark Jaquith
…as well as a link to the parent plugin on its WordPress.org Plugin home page.
If in doubt, you can always try to contact the plugin’s developer(s).