In this article, we consider the problem of detecting software that has been pirated and modified. We analyze a variety of detection techniques that have been previously studied in the context of malware detection. For each technique, we empirically determine the detection rate as a function of the degree of modification of the original code. We show that the code must by substantially modified before we fail to reliably distinguish it, and we show that our results offer a significant improvement over previous related work. Our approach can be applied retroactively to existing software and does not require access to the source code, and hence it is both practical and effective.