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).
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.