It's in whatever order they were programmed in, and they move with tire rotation, so you have to figure it out.
The easiest way I know, is to set the tire pressures apart by 3 lbs, all around, e.g. tire one, 3 lbs over, tire two at recommended, three at 3 lbs under, and four at 6 lbs under. You should then be able to identify all four positons.
Then mark them somehow, perhaps take a sharpie marker and put a small number behind a spoke on each, or whatever you decide. You can then track them as they move around during rotation.
This has been discussed before, so a search will turn up additional information, such as the default configuration for the first programming.
The sensors don't know where they are on the vehicle, so you have to track that. But they should always appear in the same order on the display, from what I've read.