A (not very useful) AI filter: Suppose you have two AIs of the same architecture, one of which has friendly values and one of which has unfriendly values. You run them through a set of safety tests which you are confident only a friendly AI could pass, and both AIs pass them. You run them through a set of capability/intelligence tests and they both perform equally well. It seems that the unfriendly AI is in a slightly unfavourable position. First, it has to preserve the information content of its utility function or other value representation, in addition to the information content possessed by the friendly AI. The unfriendly AI would also need more preparation time at some stage in order to conceal its utility function. Thus, if you measure storage and time costs, whichever AI is smaller and faster is likely the friendly one. However, I don't think this directly yields anything good in practice, as the amount of extra information content could be very small, especially if the unfriendly AI simplifies its utility function. Also, you need a friendly AI...