Currently we don't expose information about whether a connector is a source or sink. This is useful when, e.g., categorizing connectors in a UI. Given naming conventions we try to encourage you might be able to determine this via the connector's class name, but that isn't reliable.
This may be related to https://issues.apache.org/jira/browse/KAFKA-4279 as we might just want to expose this information on a per-connector-plugin basis and expect any users to tie the results from multiple API requests together. An alternative that would probably be simpler for users would be to include it in the /connectors/<name>/status output.
Note that this will require a KIP as it adds to the public REST API.