Quick Answer
Water utilities publish a Consumer Confidence Report (CCR) annually. Find the sections on disinfectant (chlorine or chloramine), hardness (usually mg/L), and any health-based standards. The two numbers that matter most for shower filtration are the disinfectant and the hardness.
Problem
Water reports are dense tables of numbers that most people skip. The key facts for choosing a shower filter are buried in jargon.
Product Recommendation
Once you know your disinfectant and hardness, matching a filter is straightforward: KDF/carbon for chlorine, vitamin C stage for chloramine. ZOOSHEN publishes media per system. See /water-science/chlorine-vs-chloramine.