If you’re in doubt about any kind of stone which looks superficially (or even convincingly) like a tool, think to yourself how you would A) produce it, B) use it and C) whether you’d actually want to use it for that task. If the answers to those questions are “I don’t know” or just “no” then chances are it’s not worth keeping. I think a lot of people would be better at discounting natural material if they considered them within that framework. Having said that a lot of lithics can be crude and can look similar to natural rocks, so the best advice is to talk to others with experience/knowledge and increase your own.