Undergraduate field experiences (UFEs) are a prominent element of science education across many disciplines; however, empirical data regarding the outcomes are often limited. UFEs are unique in that they typically take place in a field setting, are often interdisciplinary, and include diverse students. UFEs range from courses, to field trips, to residential research experiences, and thereby have the potential to yield a plethora of outcomes for undergraduate participants. The UFE community has expressed interest in better understanding how to assess the outcomes of UFEs. In response, we developed a guide for practitioners to use when assessing their UFE that promotes an evidence-based, systematic, iterative approach. This essay guides practitioners through the steps of: identifying intended UFE outcomes, considering contextual factors, determining an assessment approach, and using the information gained to inform next steps. We provide a table of common learning outcomes with aligned assessment tools, and vignettes to illustrate using the assessment guide. We aim to support comprehensive, informed assessment of UFEs, thus leading to more inclusive and reflective UFE design, and ultimately improved student outcomes. We urge practitioners to move toward evidence-based advocacy for continued support of UFEs.