Wordpress and its plugins are written in PHP. As PHP is a scripting language, so you can read the source code of the whole plugin.
People who null plugins just go through the code "line-by-line", learn how it works, find license checking functions, and modify them to never run or always give valid results.
If you are somewhat good in PHP, but not good enough to cr@ck it yourself, you can try downloading the non-nulled, same version of the plugin that you are using, and compare all the files (with a software) to the nulled, to see what is modified. Then if you are lucky, doing the same modifications to the updated plugin files will get the plugin nulled.