I was reading this question: Looking for ways to reduce trub And since I also fly sparge, I would like to know how to detect channeling in the grain bed? I always leave around 3cm of water over the grains, and when the wort is almost completely drained, the grains surface is almost flat.
The bad news is that you can't detect channeling while you are fly sparging... but you can see it afterwards. This works best in darker beers.
After the mash get something to scoop out the spent grain. A stainless steel mixing bowl worked pretty well for me. Scoop out a bit of the grain, then look at what's in the bowl and what's left in the mash tun. Look for darker sections that are the color of your wort and lighter sections that are just the color of spent grain. Repeat this process until you have emptied the mash tun.
You should be able to see lighter and darker sections that identity the channeling if it is happening.
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