Intermolecular coupling plays a key role in charge transport and excited-state dynamics in organic systems. A key example of the influence of intermolecular interactions is the process of singlet fission, which involves the production of two triplet excitons on neighboring molecules from excitation of a singlet exciton on one molecule. This intermolecular pair-production process has the potential to boost the efficiency of photovoltaics beyond the Shockley-Queisser limit and has been used to produce photovoltaics with external quantum efficiency above 100% [D. L. Dexter, J. Lumin. 18, 779 (1979); D. N. Congreve et al., Science 340, 334 (2013)]. A key challenge in the field of singlet fission has been to quantify the strength of intermolecular spin coupling. To address this question, we have applied pulsed (up to 68 T) and static (up to 30 T) magnetic fields to induce avoided spin-level crossings, which allows us to measure the exchange interaction between triplet excitons in an organic semiconductor. At magnetic fields where bright and dark pair-states mix, the excited-state emission diminishes, yielding an optical signature of the spin-level structure. Using a canonical singlet-fission molecule we extract distinct exchange values ranging from 0.4-5.0 meV (Figure). This variation in spin-spin coupling arises from the intrinsic variation in pair conformation in this material. We have further shown that the sensitivity of the exchange interaction can be used to distinguish the optical signatures of distinct pair sites. This method for quantitatively probing excitonic spin interactions presents an opportunity for understanding the role of molecular conformation in spin coupling and paves the way for molecular design of excitonic interactions in optoelectronic and spintronic devices.