How to monitor 'bad' messages written to kafka topic with no schema

I use Kafka Connect to take data from RabbitMQ into kafka topic. The data comes without schema so in order to associate schema I use ksql stream. On top of the stream I create a new topic that now has a defined schema. At the end I take the data to BQ database. My question is how do I monitor messages that have not passed the stream stage?
another question I have, in this way, do i support schema evolution? and if not, how can i use the schema registry functionality? the main goal is to validate messages schema.