How can you tell if your answer has been featured?
A little trophy thumbnail appears next to it.
This mark appears to the left of the answer:
It's a little thing, yes, but if you look closely enough you'll be able to see it.
Also, if you click "View More" under the featured answers section at the top of a subject page, you will be able to see all of the recently featured answers for that subject:
So, for example, if you are active in Calculus and would like to see if you have any featured answers in that subject, you would click "View More" and scroll down until you find a featured answer of yours.
On top of that, every time one of your answers is featured, you receive an email (at least, I do). I suppose you could also check your email account for those emails that tell you your answer has been featured. The email will look something like this:
Those are the methods I can think of right now, but if there are more please describe them in the comments or leave an answer of your own.