From the location of the peaks you get the frequency of the photons and thus their energy (which is the splitting of the hyperfine levels), and from this you get the strength of the hyperfine field, using the formula for hyperfine splitting.
EDIT: I just realised the question was about the sign of the hf field, not its strength.
You can see that for an increasing external field, the hyperfine splitting (frequency of the photons) decreases.
I don’t see how you can get the sign of the hyperfine field from this, since (how I see it) the hyperfine splitting will always decrease for increasing external field, no matter the sign of the hyperfine field.