You want to identify differences (entities, fields, picklists, etc.) between two CRM Online instances (e.g., production and a sandbox).
You might be able to follow the approach detailed on Develop 1's blog, but if your default solution zip file never appears or you get an error (as I do quite often these days with the default solution), then here's another approach.
Note that these steps are targeted toward CRM developers. That is, until someone writes an XrmToolBox plugin that does the same thing or in a better way.
Export entities as solution zip files: The Dynamics 365/CRM SDK contains an article named "Work with Solutions" that provides source code samples for creating and exporting solutions. The first step in the diff process is to write an app that creates a temporary solution in the instance, adds an entity component to it, exports the solution (as a zip file) and then moves to the next entity, and so on. This will result in all entity components (and their dependencies) in solution zip files.
Extract the solutions to sub-components: Use the SolutionPackager tool that's included in the SDK to extract each entity solution zip file to individual sub-component files. You can use Excel to build a batch file that will run the SolutionPackager utility for each solution zip file, or better yet, run the SolutionPackager from within your app.
Follow the steps above for a second CRM instance: Now that you have all entity components extracted on disk, you can run steps 1 and 2 again for a different CRM instance. Or run your app twice at the same time, one pointed to production and one to the sandbox.
Use a file diff/comparison tool to identify differences: You can now use a diff tool such as Beyond Compare to diff the two sets of folders. This will reveal metadata differences in both instances.
By exporting entities along with their required components (that's an option when adding components to a solution programmatically), you'll end up extracting most components that relate to entities. However, you might also want to export other component types such as all processes (not only those that have entity dependencies), global picklists, web resources, etc.
It might take an hour or two for the app you create to export all entities as solutions.
You can add the extracted solution files into a source control system to keep track of changes over time.
You can index and search through the solution zip files or extracted files using a grep or indexing tool (e.g., dtSearch) to track down a component that might be giving you trouble while importing or exporting larger solutions.