The reliability of introspective or first‐person reports has often been questioned. Micro‐phenomenology is a method that has been devised to increase the reliability of such reports. One way of showing that the method actually increases reliability is to test it in experiments where reporting can be checked by an external observer. Though such approaches are highly valuable, the problem remains that the actual experience cannot be investigated directly by external means. This justifies the complimentary approach of developing internal criteria for evaluating first‐person reports. Here we investigate the sense of certainty as a possible internal criterion. We employed the technique of micro‐phenomenological self‐inquiry to investigate the characteristics of this internal sense. A mind‐wandering task was conducted that repeatedly leads to situations in which certainty and uncertainty in relation to past mental events and descriptions can be experienced. These situations were explored in a number of trials until a certain set of characteristics of the sense of certainty emerged. The result is a methodical procedure to enhance the reliability of first‐person reports.