It is widely accepted that in order to deliver the best Qualityof- Service (QoS), applications need to be adaptive to the fluctuating computing and communication environments. The middleware layer may assist by controlling the behavior of the applications so that they adapt and reconfigure themselves. In this paper, we present QualProbes, a set of middleware QoS Probing and Profiling services to discover such relationships at run-time. Our approach focuses on meeting the requirements of the critical performance criterion in the application. Such criterion may be affected by changes in more than one application-specific QoS parameters, and these parameters have diversely different resource usage patterns. QualProbes services are able to precisely capture the effects made to the critical performance criterion when resource availability varies, and thus enable more effective control of the application to adapt to resource variations. Our case study with OmniTrack, an omni-directional visual tracking application, provides solid proof that QualProbes significantly enhance our capabilities to satisfy the critical performance criterion, the tracking precision, while controlling the adaptation process of the application.