I think your best shot is to look at the appropriate section of the portal. Normally, new CCM releases are a good starting point for most APIs (as they are tied to CCM versions.. ) but for IPSS there's no coordinated release schedule I'm aware off.. so besides checking for updated developer guides whenever there's a new CCM I just check the IPSS developer section every now and then for updated information.

I'd be careful about using stuff that's only in the schema but not documented.. it may not be for public consumption and you're going to be in a bind if they retire some of it.