When you feel comfortable towards that person,than anyone else and when you both are supportive with each other without needing to impress,without any competition or jealousy.Yung tipong na eenjoy mo yung company niya at you feel lucky to be with that person and you just feel freely and equally comfortable with them even in silence.Kahit meaningless convo turns out to be awesome and interesting pa din wala masyado pressure on both of you and seems everything feels better.You just tend to get attracted to them not only physically but mentally and emotionally.Yung imperfections nila it isn't a deal breaker to you and they always bring out the best version of yourself.Just learn to hear your inner voice and trust your instinct pay attention of how that person makes you feel,wether you fell really happy and healthy then that would be a great sign.