If you haven't planted a lot of indigo, it can be really easy to confuse Japanese indigo (Persicaria tinctoria, or Polygonum tinctorium) with some of its cousins in the Polygonaceae smartweed/knotweed family. Here in Indiana, there are common smartweed varieties whose leaves, stems and even flowers look just like indigo, and tend to find their way into your indigo bed. They look the same, but they do not contain indigo. It's especially important to remove them from your bed early if you are collecting your own seed, since their flowers are nearly identical.
There are a couple of key characteristics to tell other smartweeds apart from Japanese indigo.
The leaves of Japanese indigo are broader and rounder than other smartweeds (I tend to think they look a little like Genovese basil leaves). Smartweed cousins will have narrower leaves, and actually tend to be a slightly bluer green than the bright green Japanese indigo leaves.
It can be a little harder to tell when they're smaller, but you can still see the difference. Narrower leaves, a little bit bluer than the indigo.
Can you spot the smartweed in the indigo?
Some people are confused by this purple spot in the leaf, thinking this is the indication of indigo content. Nope! That's a dead giveaway that it's NOT Japanese indigo. The blue won't show up on the leaves until the leaf is dried or broken down (or damaged).
Indigo Makes Your Fingers Turn Blue
If you squish a leaf between your fingers for a couple of minutes, they should turn distinctly blue. (This is also a good way to check the health of your indigo.)
Time for a POP QUIZ!
Is this Japanese indigo, or a different smartweed?
It's a different smartweed! The leaves are a bluer green, and they are narrower and more elongated than Japanese indigo.
Hopefully this helps you in your plant identification.