Isn't my knowledge of other men's fruits, subjective? Besides, what does that really mean?
Yep. And it's based on your opinions and experiences alone, as you've stated.
The Reformed Christian believes that his knowledge, his fruits, his salvation, his every breath, are all based on God's will alone. We objectively and subjectively believe this because the Holy Spirit guides us and comforts us and assures us that God's will is subjective to nothing but Himself.
Them's the cards. Deal with it.