It's probably TPO, I know mine is, but I have a 2009 so not sure what they were using in 2013. One way to tell the difference (although not the only way) is if the roof has sort of an orange peel texture to it it's likely TPO while an EPDM roof will be smooth. A google search will likely turn up more ways to tell.
2011 Chevy Silverado Crew Cab 4X4
2009 210 WBS