As a first step towards assessing the quality of support offered online for Open Source Software (OSS), we address the task of locating requests, i.e., messages that raise an issue to be addressed by the OSS community, as opposed to any other message. We present a corpus of online communication messages randomly sampled from newsgroups and bug trackers, manually annotated as requests or non-requests. We identify several linguistically shallow, content-based heuristics that correlate with the classification and investigate the extent to which they can serve as independent classification criteria. Then, we train machine-learning classifiers on these heuristics. We experiment with a wide range of settings, such as different learners, excluding some heuristics and adding unigram features of various parts-of-speech and frequency. We conclude that some heuristics can perform well, while their accuracy can be improved further using machine learning, at the cost of obtaining manual annotations.