If the offspring has impossible moves (e.g. Nasty Plot + Hypnosis Crobat) and/or impossible DW abilities (e.g. DW Litwick, DW female starter Pokemon), then the offspring is hacked. It's going to take some research to find what exactly is impossible. I do not ever encourage hacking parents nor using hacked parents. If you do so un/intentionally and are caught, you will remove the illegal Pokemon and can get a strike depending on severity of offense. If the offspring has nothing impossible, then the offspring is legal.