It's a very good question. And ultimately we can't know because we cannot look upon another's heart. But though we can't make final judgments, we can make functional judgments and are encouraged to do so...1 Corinthians 2:15...but he who is spiritual judges all things...
So what should we be looking for?
We are told that the Spirit will lead us into all truth so while we all hold some truth in error, we should be getting closer and closer to the truth. And scripture tells us we were created unto good works so these should attend our walk. But works, too, can be misleading because Jesus told many to depart who had quite the works resume. I've personally found the most accurate indicator to struck me be the work of the Holy Spirit in creating the reality of spiritual growth. As the Spirit works a person should become more loving, experience more peace and joy, extend greater longsuffering and kindness, exercise increased gentleness, etc. In short, we should actually become more like Jesus.
Hope this proves helpful.