Did Kaori know that Arima loved her? From the letter she gave to Arima, she confessed everything and even gave that photograph, but at the same time, she told him to throw it away if he wanted to. This according to me suggests that she didn't know if Arima loved her. If she knew about Arima's feelings, she wouldn't have said it to him. Is there any strong evidence whether Kaori knew about Arima's feelings in the Manga/Anime?
I've only watched the anime (and a few months ago at that) but truth be told, I'm of the opinion that she did know that Arima had feelings for her. Take what I'm about to say with a grain of salt though as I do think that Kaori's true goal at the end was to get Arima to love himself somehow by whatever means she could. I don't think her true intention really was to get him to fall in love with her either but become a part of his life that would would galvanize his restoration.
Here are my thoughts:
Throughout the series, Kaori shows immense respect towards Arima's skills having admitted at some point later in the series that he was a source of inspiration for her. As a result she seems overwhelmingly distraught at the state of disrepair that Arima has fallen into. She initially enters his life by the first performances together and using Watari.
As this goes on its easy to see by everyone in the series that Arima has formed a dependency towards her (especially Tsubaki), and whether or not these are romantic, Kaori seems to realize this or even if she doesn't the scale of meddling and pushing Arima to strive towards reclaiming some former glory increases. Unfortunately Kaori is doomed, and then realizes that all her effort to push him to revitalize him is at stake and gives him her final wishes etc in an effort to convince him to move on with his life after her. I think this acknowledgement of dependency and her trying to help him move on from her as a result is key to the idea that she knows he loves her.
I know I'm making Kaori seem manipulative (or unknowingly manipulative) and maybe I'm putting too much confidence in what she's capable of doing but at the least that's how I interpret it. As with the shows name, she lied. Kaori told Aroma, Watari, Tsubaki, and everyone else, that she loved Watari. By telling everyone this it was known as a fact. But then you realize she loves Arima instead. Resulting in," Your Lie in April," because she lied in the month of April by using Watari.
This question eventually led me to a translation of the letter she had written for Arima.
Roughly translated, the ending part of the letter gives us an insight in what she knew and what she thought about Arima his feelings. She thought that he loved Tsubaki, or rather everyone knew that but apart from those 2. She, however, did love Arima. She factually said that at the end of the letter multiple times and asked him to remember her... if the letter reaches him. The way it was written indicates that she is "aware" about him loving someone else which means that he doesn't love her as a lover, in her eyes, but probably as a friend or musical partner.
Whether this thought from her is right or wrong is something I can't validate. If anything, I'd say that he loves her more than Tsubaki, but Tsubaki was most likely his first love though.
But to answer your question, according to the letter written by her, she was unaware about his feelings for her. As she wrote "I wonder if I made it into yours".
(It's been a while since I watched this one, watched it when it was airing)
Sources: finishing the anime and of course, the translated version of the letter.
Towards the ending of the anime, we realize that Kaori's sole motivation in life was Kousei and the love she had for him. Everything she did, including using contacts, changing her hairstyle, "liking" Watari just so that she could get closer to Kousei, was all done in mind that she may not live to continue the ballad. Keeping all this in mind and noticing that she executed her plan quite successfully, we can conclude that Kaori was intelligent enough to know that Kousei loved her. Also there were moments when Kousei told her personally that he loved her, though worded a bit differently (like that scene where they saw the fireflies).
(I just finished watching the anime and the plot is freshly imprinted in my mind. This is my honest opinion and I am quite sure of it too)
She does say that everyone knew that Arima and Tsubaki loved each other, so no, she does not know... which makes it even sadder.