This somewhat reminds me of “How do I test my encryption?” but that question was more specific than this one, which seems to be too broad in it’s current state.
Nevertheless, there’s an easy answer to your question:
- Check and verify all the security aspects you target with your cipher.
- When done, simply respect Kerckhoffs' principle and make your cipher public… that way, others can check on the security of your cipher and either confirm it’s security (which builds public trust) or they can show you what’s wrong with it.
If you need any help or have any questions while checking and verifying your cipher algorithm, please feel invited to ask more specific questions that can be answered more on-point.