I think the concern about slippage on a harmonic balancer is between the center hub and the outer ring. in many OEM they are only connected by a rubber ring that can break down over time. or if you put a strap wrench on the outer ring to turn over the motor. then the timing mark that should show TDC is actual not showing it. it is possible to confirm with a piston stop in the #1 spark plug hole. here is a good video small block Chevy sorry. but principal is the same. https://www.youtube.com/watch?v=RENhHI9n65I

edit got beat to the point looking up the video.