We present a tool BEST (Binary instrumentation-based Error-directed Symbolic Testing) for predicting concurrency violations. We automatically infer potential concurrency violations such as atomicity violations from an observed run of a multi-threaded program, and use precise modeling and constraint-based symbolic (non-enumerative) search to find feasible violating schedules in a generalization of the observed run. We specifically focus on tool scalability by devising POR-based simplification steps to reduce the formula and the search space by several orders-of-magnitude. We have successfully applied the tool to several publicly available C/C++/Java programs and found several previously known/unknown concurrency related bugs. The tool also has extensive visual support for debugging.