Captchas are used as a security mechanism on the web to distinguish human users from automated programs. However, existing captchas are not well-adapted to mobile devices and may lead users to abandon tasks. Although Web developers have many available captchas, they lack the tools to evaluate if these captchas are suitable for their mobile site. In this paper, we present domain specific usability heuristics for evaluating captchas on smartphones. To assess effectiveness, we compared our proposed heuristics against Nielsen's during evaluations of four captcha schemes on smartphones. The custom heuristics revealed more major problems and more detailed feedback on the problems than Nielsen's.