Single-molecule force-extension experiments are an emerging tool for the study of biomolecules. For a molecule like RNA that has to fold into a specific structure in order to perform its biological function a crucial question is if such experiments can reveal this structure. I will show how the polymer physics of the backbone, the statistical physics of RNA folding, and the very detailed knowledge of the relevant free energy parameters can be combined into a computational model of such force-extension experiments. This model quantitatively reproduces experimental results for short molecules. However, for long molecules it shows that simple force-extension measurements are not able to reveal the structure of an RNA molecule due to a self-averaging effect. I will discuss how this obstacle can be overcome by including a nano-pore in the experimental setup.