Wordpress - API check for PAID plugin updates?

I'm putting together a script to check for the latest versions of every Wordpress plugin that's installed on any of my sites.

Plugins in the Wordpress repo aren't an issue, thanks to this code to fetch plugin details using the Wordpress API.

However, I can't work out how wordpress polls for updates to paid for (non API) plugins. Plugins like Gravity Forms and Advanced Custom Fields Pro.

Is there a standardised approach for these? Or could each paid-for plugin use its own mechanism to check for updates? I haven't been able to find anything in the docs, or searching.

via Chebli Mohamed