To begin with, an add-on you find aren't different from any other code snippet you find online. So these general Q&A's apply:
To answer the question:
When its not explicitly stated, you can't assume anything regarding the license of code you find.
(Blender related or not).
Contact the author and ask them to either...
- include a license header of the source code
(convention for all Blender's code).
- include
LICENSE
file in the source code repository.
(also valid but can make tracking down the license harder if files are re-arranged or re-used between projects).
Doing so will save others this hassle in the future.
Note that even in the case where the license is open-source and compatible with Blender, you may want to know the copyright holder(s) later on, especially if this ends up being used in larger projects.