We propose a methodology for non-intrusive design of concurrently self-testable FSMs. The proposed method is similar to duplication, wherein a replica of the original FSM acts as a predictor that immediately detects potential faults by comparison to the original FSM. However, instead of duplicating the complete FSM, the proposed method replicates only a minimal portion adequate to detect all possible faults, yet at the cost of introducing potential fault detection latency. Furthermore, in contrast to concurrent error detection approaches, which presume the ability to resynthesize the FSM and exploit parity-based state encoding, the proposed method is non-intrusive and does not interfere with the encoding and implementation of the original FSM. Experimental results on FSMs of various sizes and densities indicate that the proposed method detects 100% of the faults with very low average fault detection latency. Furthermore, a hardware overhead reduction of up to 33% is achieved, as compared to duplication-based concurrent error detection.