How to sync fonts across Macbooks? My old MacBook has many fonts that I need to sync to my new MacBook.  How do I figure out the ones that are missing, and sync them?
Introduction
The Fonts included with the macOS operating system have not changed a whole lot over the years. As new (and highly customisable) fonts are made available to download and add to your FontBook, it indeed becomes difficult to remember which ones you have installed yourself and which were installed by the last macOS update. Having faced this issue for several years now, I  believe that I have found and implemented the best solution implemented across my various mac devices.

The Concept
I have discovered that the following solution is both reliable and very easy to set up. It includes uploading the entire fonts folder on the mac to Dropbox. Already being a frequent user of the platform, I am a big believer in syncing my fonts this way because my Design projects live there permanently. The big advantage is that the work and fonts are all in one location (Dropbox) from where I can access them at any time without having to search the internet for that one awesome font I haven't installed locally on the end-device. If I share a project, I can also have instant access to include the fonts in the project files, making it super easy for the receiver of my work to continue.

The Setup
You can find these sorts of guides all over the internet but I have found this one to be the most reliable in terms of macOS changes.

Your Mac:  


Launch Terminal in Applications/Utilities/
Run cd ~/Library to open your user Library.
Run sudo mv Fonts ~/Dropbox to move your Fonts folder to your Dropbox.
Enter your password when prompted.
Run ln -s ~/Dropbox/Fonts to create a symbolic link in your User Library.


Target Mac:  


Launch Terminal in Applications/Utilities/
Run cd ~/Library to open your user Library.
Run sudo rm -r Fonts to delete your user fonts folder.
Enter your password when prompted.
Run ln -s ~/Dropbox/Fonts to create a symbolic link in your User Library.


Things to consider
While this setup works well for my own purposes, you may like to thing about using a different cloud storage provider such as Google Drive or Apple's iCloud, which may work even better on other Apple devices.
Let me know how you make out and whether you have any questions about this method in action. I am sure it would fit your needs and I'd be more than happy to follow up with you on this.

Follow-up Additions  


This method essentially removes all Fonts from the mac and migrates them to Dropbox, then a link to the location is created, allowing any of your linked devices (or through the web interface) to add new/delete old fonts to/from your "repository".  
After the initial synchronization between Dropbox and the mac, you are able to disconnect from it because the font files have been synced to the Dropbox folder in your file system, much like every other cloud storage solution has that offline component. You may make changes to any content, once you connect to the internet, it syncs up and any changes push to all devices.  
If you are a one-off solution, I'd recommend not going to all the trouble of creating the symbolic link etc, and simply copying all the fonts to a folder on Dropbox or any other physical/cloud storage medium, deleting all fonts in the Fonts folder on the target machine and pasting all of them into the (now empty) Fonts folder.  
If you wish to keep existing ones, I have honestly not looked into that in too much detail as it wasn't applicable to my situation at the time. Apps like Compare Folders work really well when locating differences. I would suggest you check the source mac's Font folder against the target mac's Fonts folder and view the differences. This would allow you to pick out those that are not in the target folder. It isn't all as complicated as it sounds.