Hi Leon, welcome
You need two things, de-ionised water and some pH testing paper.
Make sure your hands are clean and dry, not wet with any cleaning chemicals.
Simply add a drop of de-ionised water to the carpet, tear off a bit of testing paper and push the paper into the dampened carpet with something like a pencil or pair of scissors.
You should see the paper dampen, if it doesn't add a little drop more water.
The reading is instant, just compare the colour of the paper with the dial on the dispenser.
Why do you need to know this, you having a problem with a carpet you've cleaned?