  Recent advances in high-throughput experimentation for combinatorial studies
have accelerated the discovery and analysis of materials across a wide range of
compositions and synthesis conditions. However, many of the more powerful
characterization methods are limited by speed, cost, availability, and/or
resolution. To make efficient use of these methods, there is value in
developing approaches for identifying critical compositions and conditions to
be used as a-priori knowledge for follow-up characterization with
high-precision techniques, such as micron-scale synchrotron based X-ray
diffraction (XRD). Here we demonstrate the use of optical microscopy and
reflectance spectroscopy to identify likely phase-change boundaries in thin
film libraries. These methods are used to delineate possible metastable phase
boundaries following lateral-gradient Laser Spike Annealing (lg-LSA) of oxide
materials. The set of boundaries are then compared with definitive
determinations of structural transformations obtained using high-resolution
XRD. We demonstrate that the optical methods detect more than 95% of the
structural transformations in a composition-gradient La-Mn-O library and a
Ga$_2$O$_3$ sample, both subject to an extensive set of lg-LSA anneals. Our
results provide quantitative support for the value of optically-detected
transformations as a priori data to guide subsequent structural
characterization, ultimately accelerating and enhancing the efficient
implementation of $\mu$m-resolution XRD experiments.
