  Based on the occurrence rates implied by the discoveries of 1I/`Oumuamua and
2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time
(LSST) should detect $\ge1$ interstellar objects every year (Hoover et al.
2021). We advocate for future measurements of the production rates of H$_2$O,
CO$_2$ and CO in these objects to estimate their carbon to oxygen ratios, which
traces formation locations within their original protoplanetary disks. We
review similar measurements for Solar System comets, which indicate formation
interior to the CO snowline. By quantifying the relative processing in the
interstellar medium and Solar System, we estimate that production rates will
not be representative of primordial compositions for the majority of
interstellar comets. Preferential desorption of CO and CO$_2$ relative to
H$_2$O in the interstellar medium implies that measured C/O ratios represent
lower limits on the primordial ratios. Specifically, production rate ratios of
${\rm Q}({\rm CO})/{\rm Q}({\rm H_2O})<.2$ and ${\rm Q}({\rm CO})/{\rm Q}({\rm
H_2O})>1$ likely indicate formation interior and exterior to the CO snowline,
respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior
to the CO snowline. We provide an overview of the currently operational
facilities capable of obtaining these measurements that will constrain the
fraction of ejected comets that formed exterior to the CO snowline. This
fraction will provide key insights into the efficiency of and mechanisms for
cometary ejection in exoplanetary systems.
