An X-Ray is generally not very helpful for Pectus Excavatum evaluation. The most common measure of severity of Pectus Excavatum is the Haller index, which can only be obtained reliably from a CT scan.
There is ongoing research to determine the efficacy of using X-Ray to measure the Haller index, but the gold standard today is to use CTs. In some cases, a lateral X-Ray may be used to evaluate the Haller index. This is because the sternum (breast bone) may be seen if the indent is severe enough. Otherwise, the sternum is hidden by the ribs in the lateral view. See this website for example images.