Four things that instantly tell you the last person managing this account wasn't checking anything

Wasted spend rarely shows up as an obvious red number. It's usually sitting inside numbers that look fine on the surface.

Start with break even ACoS per ASIN, sale value minus COGS divided by sale value, to get the real profitability ceiling instead of guessing at a target. From there, compare ad attributed conversion rate against the account's overall purchase rate in Search Query Performance, a gap between the two usually means paid traffic with weak intent is being paid for. Last step is pulling the 30-day search term report and checking for root level negative keywords that are quietly blocking exact searches that would have converted well.

Each step alone catches something different. Run all three and the waste usually isn't hiding in one place, it's spread across all of them.