You cant rely so much on the A/F meter alone during tuning. Instead, a knock monitor toy should be working hand in hand with your chosen A/F meter.
HKS (Old Model)
The latest HKS A/F Knock Amp has both capabilities of a A/F meter and Knock Monitor. Apexi Power-FC has knock meter on its handheld commander.
So far the best knock monitor i think would be the :
I believe by listening and looking at the pattern using a software would help even better to detect knocking.