Both PVC and TPO membranes are typically white in color with heat welded seams. They are also applied in similar ways. The best way to determine the membrane technology is to look at the project file or warranty that was issued. Another way to determine the membrane type is to see if the membrane is embossed/ marked with the manufacturer’s name and/or production date/info.  If those options for determining membrane type do not work, a sample of the membrane should be taken and sent to your roofing manufacturer of choice, or a roofing consultant and they can help to determine which technology it is.