Determining that a schema has changed for purpose of monitoring

Hello.

Is there a way in PostgreSQL for a client to determine that a database schema may have changed since it checked it earlier, for example to get the transaction ID or timestamp of the last successfully committed transaction that included DDL commands and compare them over time? For a while I've also seen plans and bits of work related to DDL triggers that might help me but don't have track of where this stands today.

The background is that I've created a runtime application testing tool and that information would be useful to me, as it's currently manually triggered and I'm looking to explore the possibility of making applications capable of performing non-invasive self-testing with minimal configuration or work needed, at first to the extent of verifying that SQL statements parse and that the types and constraints of their fields don't change in a way that could cause problems for the application.

Thanks,
Thor Michael Støre