How to detect spyware

I have read that some spyware can survive a factory reset. I would like to detect first if I have spyware and I want an app that is free. And it not spyware if it can’t detect it so there has to be accurate anti spyware out there. I don’t want a reply asking why I want the spyware or what’s the problem. Just advise on apples support for spyware detecting besides the factory reset.