Among platform designers there is still little actionable knowledge about how to foster community engagement, and new community platforms are likely to fail. Much research has documented effects that could inform better designs, however there is a lack of practical methods to review particular concerns. We design a lightweight method for systematic reviews of community engagement scenarios, based on a catalogue of 59 techniques, presented in the form of method cards, and co-developed with community engagement experts. We validate it in problem-solving workshops with organisers, and find that a full review is feasible in under 15 minutes, that it provides effective prompts and important guidance, and can successfully support process innovation. Finally, we validate it for use in summative evaluations of online communities. In a large-scale observational study of 1 million Flickr users, application of the method reveals the surprising impact of a major site redesign: it successfully increased content engagement, but at the expense of social interactions.