Tail rot will look like any other infected wound. The same as if you got a scrape or cut and it became infected. The difference with it happening in beardies tails is that there is not much flesh in the tail, so any infection in the tail can easily start killing all of the flesh in the tail as it eventually starts to work its way up into the deeper and thicker parts of the tail.
In those case, the tail will start becoming darker and black as the flesh dies. Not to be confused with beardies having the natural ability to darken their tails just like they can with their beards and the rest of their body.
Here is an example of tail rot. Note the obvious damage to the flesh compared to the healthy tissue around it. It's not the color of the tail that dictates tailrot, it's the infection and obvious signs outside of the color.
-Brandon