This study aimed to develop and validate, based on the Evidence Centered Design approach, a generic tool to diagnose primary education pupils' prior knowledge of technological systems in primary school classrooms. Two technological devices, namely the Buzz Wire device and the Stairs Marble Track, were selected to investigate whether theoretical underpinnings could be backed by empirical evidence. Study 1 indicated that the tool enabled pupils to demonstrate different aspects of their prior knowledge about a technological system by a wide variety of work products. Study 2 indicated that these work products could be reliably ranked from low to high functionality by technology education experts. Their rank order matched the Fischer-scale-based scoring rules, designed in cooperation with experts in skill development. The solution patterns fit the extended non-parametric Rasch model, confirming that the task can reveal differences in pupils' prior knowledge on a one-dimensional scale. Test--retest reliability was satisfactory. Study 3 indicated that the diagnostic tool was able to capture the range of prior knowledge levels that could be expected of 10 to 12 years old pupils. It also indicated that pupils' scores on standardised reading comprehension and mathematics test had a low predictive value for the outcomes of the diagnostic tool. Overall, the findings substantiate the claim that pupils' prior knowledge of technological systems can be diagnosed properly with the developed tool, which may support teachers in decisions for their technology lessons about content, instruction and support.