Testing for chroma subsampling is very easy. Just open up a test pattern, then observe it and check if any of the lines and text are blurred together.
If the text does not blend together and shows artifacting, then the TV and mode you are using does not use chroma compression and is showing chroma 4:4:4.
It’s also important to make sure that Windows scaling is set to 100%. With larger, high-resolution displays, it is common for Windows to scale UI elements at 150% or more, and this can cause false positives when testing for Chroma Subsampling.