Despite its widespread use, post-tensioning (PT) technology still presents significant challenges when it comes to accurately assessing the condition of PT systems. The inherent characteristics of post-tensioned structures limit the applicability of conventional investigation techniques and visual inspections typically used for reinforced concrete structures. As emphasized in the Italian Guidelines for the assessment of existing bridges, evaluating post-tensioned bridges is primarily a matter of safety. A key concern is the loss of prestressing force, which may result from both long-term effects related to material creep and degradation phenomena affecting the PT tendons. Such losses can compromise the structural capacity of the bridge and potentially lead to brittle failure.Destructive methods are generally discouraged for assessing residual prestress in PT structures, as they may further damage the system. This study focuses on the Flat-Jack Test, a minimally invasive technique that introduces limited damage to the structure and is relatively simple and practical to perform in situ, both in terms of execution and required equipment. While the test is well-established and standardized for masonry structures, its application to concrete—particularly post-tensioned concrete—remains uncommon and not yet fully validated.The paper presents the results of an experimental laboratory investigation in which the Flat-Jack Test was applied to large concrete blocks subjected to known, controlled stress conditions. The test’s accuracy in estimating the actual stress state was evaluated. One of the primary factors affecting the method’s reliability is shown to be the difficulty in accurately determining the effective area of the cut made in the concrete to insert the flat jack. Additionally, the paper presents some findings from Flat-Jack Tests conducted on real bridges, highlighting the practical challenges and limitations of applying this method to post-tensioned concrete.