With only one sensor, its value is as accurate as can be: I got no other value to compare to.
Since I got a BME280 and a DS18B20 and both can measure temperature, I have something to compare. Both have a quite high resolution (0.01 resp. 0.0625 degree), but less accuracy (±0.5 degree at 25°C or ±1 degree from 0…65°C for the BME280, ±0.5 degree from -10..+85°C for the DS18B20), I was curious how they compare. Here is the result:
The upper one is the BME280, the lower one is the DS18B20. Difference is about 0.75 degree.The truth is probably somewhere in the middle, but hard to say without proper calibration. Unfortunately the BME280 is for air usage only which makes calibration against e.g. ice water or any other liquids challenging.
I have another DHT22 (AKA AM2302) sensor , which shows temperatures between above. Unfortunately its humidity sensor seems to be broken.