Monitoring systems for malicious behavior increasingly requires aggregating and analyzing data from various sources, such as network flows, host logs, and end-point monitoring platforms. However, there's currently a lack of metrics and methodologies to compute the observability and efficiency of a security monitoring strategy. This manuscript introduces TOMATO (Threat Observability & Monitoring Assessment Tool), which is a platform to evaluate the effectiveness of a security monitoring strategy by exploring both the number of known adversarial techniques that can be detected within a network, along with evaluating the number of false-positives produced by the monitoring strategy. The output produces both an observability score and efficiency score of a set of deployed monitoring techniques, which are evaluated based on the data from the environment, and simulated attacks generated from MITRE ATT&CK. The proposed approach is then integrated into an ELK stack and evaluated on real SCADA devices within the WSU Smart City Testbed.