How do I know when I've found the right prom dress?

It doesn't matter. In a few years from now, you'll look a pictures and wonder what you were thinking. 

Olivia, I'm going to tell you the truth.  Are you ready?

There is no such thing as "the right prom dress."

Everyone's tastes are different and, if you are like most of us, your own tastes change from month to month, week to week, and maybe even day to day.  What seems like the perfect prom dress (or perfect anything else) right now may or may not seem perfect in the future.  

What I recommend is this: after you've almost decided and are trying to choose between, let's say, three different dresses, bring a couple of your best friends along to help you decide.  And, if you are close to your Mom, bring her, too.  Maybe even have lunch out together and make a whole celebration out of it.  The purpose of doing this is to bring a sense of support and fun to whatever dress you get.  Then, even if you decide, after you've already purchased the dress, that it was not the best choice after all, you can shake off that feeling because your team is behind you, supporting you all the way.  

Here's something else to think about.  When you are fifty, you are going to look at the photos someone took of you in that dress.  No matter what dress you get, you are going to say some version of, "Oh my goodness!  Look how old-fashioned that dress was!  Look how strange my hair-do was!  And that make-up job?  Horrible!!!!!!!!  Your tastes will have changed, and the good news is that you will laugh and laugh and laugh at yourself.

So, by all means, get the dress you like the most, but try not to worry about it.  Worry won't help you have a great time at the prom.  And having a great time is really what it is all about, right?