Rev. 50 (which is the latest version, I believe) does not include source code for the OVAL checker
The OVAL Checker is just a set of schematron rules that you run against your OVAL Definition to check for common mistakes and conformance to accepted content authoring conventions. It does not have a java wrapper application like OVAL Splitter or OVAL Merge which is why it doesn't have any source code. Are you running into issues with it because you do not have a way to apply the schematron rules to your OVAL Definitions?
Also, you may find the following links about schematron validation and OVAL Definitions content authoring guidelines helpful.