Nanoencapsulated drug delivery to solid tumors is a promising approach to overcome the pharmacokinetic limitations of therapeutic drugs. However, encapsulation leads to complex drug biodistribution and delivery making analysis of delivery efficacy challenging. As proxies, nanocarrier accumulation or total tumor drug uptake in the tumor are used to evaluate delivery. Yet these measurements fail to assess the delivery of active, released drug to the target, and thus it commonly remains unknown if drug‐target occupancy is achieved. Here, an approach to evaluate the delivery of encapsulated drug to the target is developed, where residual drug target vacancy is measured using a fluorescent drug analog. In vitro measurements reveal that burst release governs drug delivery independent of nanoparticle uptake, and highlight limitations of evaluating nanoencapsulated drug delivery in these models. In vivo, however, the approach captures successful nanoencapsulated delivery, showing that tumor stromal cells drive nanoparticle accumulation and mediate drug delivery to adjacent cancer cells. These results, and generalizable approach, provide a critical advance to evaluate delivery of encapsulated drugs to the drug target—the central objective of nanotherapeutics.