As health news stories affect the audiences' decision and behavior, readers need to understand and evaluate the information. However, assessing the quality of health news stories is challenging for non-expert readers, as it requires both media and scientific literacy. In this paper, we design a web interface, ReviewAid, that guides online readers' evaluation of health news stories with the evaluation criteria designed for non-experts. ReviewAid scaffolds readers' evaluation process by providing 1) explicit guidance that asks readers to distinguish the quality of media and scientific findings and 2) example comments for each criterion. Results from our study with 44 participants show that ReviewAid helps readers better connect the evaluation criteria to the specific news story. ReviewAid also enabled participants to conduct a more grounded and accurate evaluation of the content of scientific research.