Turn heating off, close doors, then measure how much the room cools.
Tip: the longer the test, the better (it reduces sensor noise and averages out short interruptions).
Benchmark source: tado° analysed ~80,000 UK homes (Dec 2019–Jan 2020) and found a typical home at 20°C indoors and 0°C outdoors drops ~3°C over 5 hours (≈0.03 per hour). Source
This is a real-world comparison tool, not a lab test. It reflects the room where your thermostat is, not your whole home.