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
DDLcommands and compare them over time? For a while I've also seen plans and bits of work related to DDL triggers that
mighthelp 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'scurrently manually triggered and I'm looking to explore the possibility of making applications capable of
performingnon-invasive self-testing with minimal configuration or work needed, at first to the extent of verifying that
SQLstatements parse and that the types and constraints of their fields don't change in a way that could cause problems
forthe application. 

Thanks,
Thor Michael Støre