It's pretty hard to hear detonation in a HotRod; everything else is louder.
Because of that I bought a G-tech-ProSS , which is a windshield mounted accelerometer synchronized to time. With this tricky little unit, you can do speed tests at various PowerTiming settings. When the car slows down, she's complaining. Catch it early.
It has some other useful features as well. If you measure out a quarter mile, and follow the prompts it will calculate your ET and estimate your power and even spit out a downloadable, dyno type graph.
Save your engine, install an accelerometer. Nice tool.