Whatever the case, if your Mac Pro shipped with a single processor, however many cores it has, then you don't have a place on the logic/main board for a second physical CPU. Unless I’ve missed something over the years, multiprocessor systems as with some Mac Pro versions will not function if only one CPU is fitted.
Steps 19–21 of a Mac Pro teardown on iFixit is your definitive visual reference check if you’re still feeling some doubt about what you have.
For your setup, iStat Menus will show, depending on your configuration settings, either all logical cores as discrete graphs/percentages or as an amalgamated single physical processor.