As assistive technology applications are increasingly implemented in early childhood settings for children who are at risk or who have disabilities, it is critical that teachers utilize observational approaches to determine whether targeted assistive technology-supported interventions make a difference in children's learning. One structured strategy that employs observations and which has powerful child progress monitoring implications is the concurrent time series probe approach. Requiring multiple performance measures of a child engaged in a targeted task over time--both with and without a specific assistive technology device--the concurrent time series probe approach can be used to evaluate the effectiveness of assistive technology tools in supporting skill acquisition in the classroom. This approach is described in the context of a case study, with accompanying explanations of how to interpret data and make decisions regarding the effectiveness of the technology. (Contains 5 figures.)