That’s stronger than Detect Evil. Because historically (as of 3.5 at least) Detect Evil isn’t quite “Know Alignment”. It detects evil auras but the only things to have evil auras, as a mortal, are powerful clerics/blackguards, or individuals steeped in actions so heinous that it actually taints them, or at least their aura, by proximity. (Which some magic items and curses can do, so even then its dangerous to overly trust the ability.)

That said, have him stab the mayor who’s evil because he’s greedy and selfish and borderline abusive in trade-deals with neighboring regions but is otherwise beloved (and has rewards heaped on him) because he’s so good at actually keeping order in the town and keeping their goodwill (although probably at least a little bit through some passive-aggressive blackmail). That’s always fun.