Two layers of underlayment are required for low-slope roofs. Take InterNACHI's Free Online Roof Inspection Course Mastering Roof InspectionTypes of Roofing Underlayments Underlayment Inspection on Roofs Fall Arrest Systems Roofs (Consumer Directed), More Inspection Items Like This. But it is easy to determine if there is only a single ply or double ply installation of roofing felt by looking at the amount of overlap of the roofing felt layers. However, the roofer was not stretching his felt in this job, and this roofing felt installation is perfectly correct.
My previous illustration, adapted from Steve Bliss' Best Construction Practices, is discussed in more detail in ROOFING UNDERLAYMENT BEST PRACTICES. In my opinion, two layers of felt over the roof liner would not significantly improve the leak resistance of a low-slope roof with asphalt shingles. Part of the problem is that if water penetrates the first layer of the roof, it can travel some distance before penetrating the second layer of the roof. Water from the felt can be absorbed by the roof cover, which can cause expansion and contraction problems of the deck.
My photo of an asphalt roof installation in process shows that the contractor has installed a peel and stick ice and water impermeable shield type product along the roof eaves in a 36 wide strip. To inspect for the presence of double-deck roofing felt, you should inspect a representative area of the roof that is accessible, such as at a gable end or on the eave (bottom edge). Your best view is from the gable end of the roof, as there you can see how the double cover felt would have been installed. Most peel-and-stick roofing membranes are designed to be used as a subfloor to be covered with a finishing roofing material, but some, such as the white MFM Peel and Stick Self Stick roofing material, which is often used in mobile homes, are designed to be left exposed to the elements.
In many cases, potential buyers skip the home altogether to avoid the hassle or require the seller to reduce the cost to cover a new roof.