The published regression corpora, run against the engine in the page: forward generation and structure round trips over thousands of structures, with every mismatch listed.
Convert a structure to its InChI and InChIKey and back, convert a whole CSV, XLSX or SDF file, learn how the identifier is derived, and practise writing its layers. The InChI engine is WebAssembly running in your browser, so the tool needs JavaScript — and nothing you open ever leaves your machine.