One telltale sign is always who they send us to for backup for their beliefs. Most often its some catechism, confession, or some mans writings with only a smattering of scripture if any.
IMO, Adam/Eve's first mistake was to listen 'another' voice. They should have kept with God ALONE.
I praise and thank God for His Word and His Spirit to teach me and bear witness to my spirit. He equipped us well. Others can listen to 'another voice' all they want. But for me and my house - God's Word is The Final Authority as it is for all born again believers.