How does one discern between thoughts inspired by the Holy Spirit and those produced from themselves

This question has been nagging at me for a while, how can one tell the difference between honest-to-God inspiration from the Holy Spirit and thoughts that are induced by themselves out of the needs or fears of the person. The Spirit is described as a 'still small voice' which can be strikingly similar to the things you tell yourself if you want to convince yourself of something. Example: I have this friend who went on about three dates with a guy before he proposed to her claiming he had prayed to God and received revelation from the Spirit that they were meant to be together. She of course didn't buy into it and rejected him flat out, because he had convinced himself of revelation that was simply untrue, no matter much he wanted it to be. How can we prevent ourselves from making similar mistakes?