I wanna congratulate my gf on her exam

I don't know if she will actually pass, but I wanna express that I believe in her and that I know she did her best. I was thinking something like:

"おつかれさまでした! You're the smartest girl I have ever known, I love you."

Any help in translating this correctly is appreciated.

You might not have to make it super specific with her and just to say something along the lines of "おめでとう. 愛してます."

Basically just saying "awesome job! I love you" which would still be very personal.

Thanks Tom, I'll use that.

If you don't know if she's actually passed, おめでとう might give off the opposite impression, almost like you're congratulating her for simply having been able to complete the test's questions.

If you want to show that you know she did her best and believe in her, you could say something like
がんばったな！きっとうまくいく。
which translates to "You really put a lot of effort into it, and I think you'll do well [on the results]."

If the OP is not a male, or looking for a unisex expression,

がんばったね！きっとうまくいく。