Accurate self-regulation of performance is important for trainees. Trainees rely on cues to make monitoring judgments to self-regulate their performance. Ideally, cues and monitoring judgements accurately reflect performance, as measured by cue diagnosticity (the ability of a cue to predict performance) and monitoring accuracy (the ability of a monitoring judgement to predict performance). However, this process is far from perfect, emphasizing the need for more accurate cues and monitoring judgements. Perhaps the mental effort of a task could be a cue used to inform certainty judgements. The purpose of this study is to measure cue utilization and cue diagnosticity of mental effort and monitoring accuracy of certainty for self-regulation of performance. Focused on the task of ECG interpretation, 22 PGY 1-3 Internal Medicine residents at McMaster University provided a diagnosis for 10 ECGs, rating their level of certainty (0-100%) and mental effort (Paas scale, 1-9). 220 ECGs completed by 22 participants were analyzed using path analysis. There was a negative moderate path coefficient between certainty and mental effort (ß = - 0.370, p < 0.001), reflecting cue utilization. Regarding cue diagnosticity of mental effort, this was reflected in a small negative path coefficient between mental effort and diagnostic accuracy (ß = - 0.170, p = 0.013). Regarding monitoring accuracy, a moderate path coefficient was observed between certainty and diagnostic accuracy (ß = 0.343, p < 0.001). Our results support mental effort as a cue and certainty as a monitoring judgement for self-regulated performance. Yet, reported correlations are not very high. Future research is needed to identify additional cues.