office-addin-manifest

This package provides the ability to parse, display, modify, and validate the manifest file for Office Add-ins.
link - npmjs.com/package/office-addin-manifest
link - github.com/OfficeDev/Office-Addin-Scripts/tree/master/packages/office-addin-manifest

Installation

Install the module by running the following command.

npm install -g office-addin-manifest 

Validation

office-addin-manifest validate <manifest> 

Yeoman Generator

If you used the Yeoman Generator to create your Office Add-in it will come with a validation module.
Your add-in project must have been created by using Yeoman generator for Office Add-ins version 1.1.17 or later.
Your add-in project must have been created later than October 2018.
When you use the Yeoman Generator to create an Office Add-in project a lot of dependecies will be installed.
One of these dependencies is the office-addin-manifest
The package.json file will contain the following entries:

{ 
   "name": "office-addin-taskpane.js", 
   "version": "0.0.1", 
   "scripts": { 
      "validate": "office-addin-manifest validate manifest.xml", 
   }, 
   devDependencies": { 
      "office-addin-manifest": "1.5.0", 
   } 
} 

Run the following command in the root directory of your project:

npm run validate 

XSD Validation

You can also validate a manifest file against the XML Schema Definition (XSD) file directly.
This includes checking all the namespaces.
Install tar and libxml

xmllint --noout --schema XSD_FILE XML_FILE