Methods and apparatus are provided for passive mid-stream monitoring of real-time properties. A passive mid-stream monitoring process is disclosed that determines whether a system is faulty. The passive mid-stream monitoring process obtains a real-time correctness property and a passively monitored mid-stream observation of the system. A timed correctness property, A.sup..sigma., is constructed from the passively monitored mid-stream observation. An intersection of the real-time correctness property and the timed correctness property is then determined to determine if the system is faulty. A passively testable determination process is also disclosed that determines whether a real-time correctness property for a system is passively testable. A determination is made as to whether (i) a set of all timed traces that are correct according to the real-time correctness property is timed prefix and timed suffix closed; and (ii) a system would satisfy the real-time correctness property if all timed-trace behaviors of the system would be included in the set.