1) If only one person knows "the secret" then it is almost certainly wrong. The easiest person to fool is yourself. We all do it (and deny it).
2) Work in groups. Invite other to collaborate fully and tell them everything you are thinking. Even a non-technical person who you trust (spouse/partner?) may easily expose a logic trap.
3) Invite outsiders in very occasionally and only for a purpose (your sponsor requires it?). Formal visits take up a lot of time and effort, and a negative (or even null or non) report will do you more harm than any likely technical contribution received*.
Absolutely spot on. Anybody would think you did this stuff before!
ETA -Agree about the approaches, even without your rep I get 5-6 a year for the last 10 years or so.