This paper presents a built-in SEU sensor (BISS) to detect soft errors in CMOS digital systems. BISS detects SEU-induced soft errors by monitoring the meta-stability in the flip-flops. BISS includes positive pulse generator, footed dynamic inverter and keeper. SPICE simulations validate the approach. Experiments show minor overhead in terms of area. BISS can yield 80% error coverage at the cost of 22% area overhead. As its prominent advantage, insertion of BISS will incur minimal performance degradation.