Question: How do you know your mics are correctly routed? Another question: Why do you need to know your mics are correctly routed?
The answer to the second question is that if you don't know which mic goes to which track of your DAW, or to which channel of your mixing console in a live sound situation, then at best you are creating a problem for yourself that will take a long time to sort out later. At worst, in live sound, you're going to wreck the gig.
Suppose therefore you are miking a drum kit with eight mics. One for each drum, one for the hi hat (just in case you need it) and two overheads. You could carefully connect the mics one-by-one, and carefully label the tracks or channels. That's what you should do. But it is surprisingly easy to make a mistake, so you need to check the connections when you're done.
The traditional professional method is to have the engineer seated at the DAW or console as his or her assistant identifies each mic in turn. The engineer uses the solo button to check the signal coming from the mics. Ideally this should be a simple matter of hopping from track to track, or channel to channel, taking less than a minute to perform, or perhaps a little longer if two or more of the connections have been accidentally swapped around.
But it's not enough for the assistant just to speak into each mic. When the mics are close to each other, it is difficult for the engineer to know for sure which mic is being spoken into. The solution is for the assistant to scratch the grille of each mic gently while identifying it. Like this…
Assistant: “Scratch scratch – This is the AKG C414 – scratch scratch – which is the left overhead – scratch scratch.” (continue as necessary)
Engineer (when satisfied): “Next”
Scratching the grille allows the engineer to identify each mic absolutely conclusively. Once done, the session or sound check can proceed smoothly. Left undone, there's a problem lurking that is going to cause immense trouble somewhere down the line.
Here's a quick demonstration with two mics. You must listen on studio monitors or headphones. You won't be able to hear this properly on laptop speakers.
In summary, this simple scratch test is quick and easy, and can save you a lot of headaches later.