I can't tell if there is fluid in my knees or not. They're both a bit swollen and pretty stiff and one is hard to lift and bend. My doctor told me that I had to go in and get them drained if they filled up with fluid so they could test the fluid to try to give me a diagnosis.
If you have fluid in your knees which I have had, when you relax your leg and rest it up on a pillow, you will feel that when you move your kneecap with your hand, it feels mushy and moves easily. You will know, it's a definite odd feeling and you can tell by looking. I hope that was helpful. The other folks here may have better or more detailed explanation for you based on their experiences.