Most Central Vacuum System (CVS) piping is 2" O.D. PVC that us labeled ASTM F-2158 for central vacuums. It incorporates swept elbows and tees so the broad radius bends don't clog. Also, the fittings tend to be directional again to minimize lint buildup. Look for low voltage control wires and also grounding wires because dust flying through PVC creates a lot of static electricity.

Another way to tell is to line up the vertical risers in the attic with the terminal locations in the home. They usually run straight up a stud bay to the attic or down into a joist bay to a basement or garage cannister.
HTH

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