I'm really struggling to answer this one

You want some form of measurement to tell you how wet a carpet is?
There are hand held probe machines out there but all they will tell you is if it's dry/damp/wet.
You could try a humidity meter. Place on carpet and take a % reading, nip outside and take a reading of the air humidity. If the outside is anywhere near what the inside is then it's going to take a while to dry. Alternatively do a flood course where they will teach you all this in far more detail.
And as you have a TM surely you should be able to get any carpet 'nearly' dry anyway.
I do remember a test done with a meter by Ken (NCCA Chap fraid my memories gone ) and I think he said it took three days for carpet to be 100% dry