For the non-cooks in the audience the dry toothpick test is used in baking (cakes, banana bread, etc.) to determine when the item is done baking. The procedure is to stick a toothpick into the food, pull it out and look at it, If it came out moist, you are not done cooking. It is simple and effective, but you can't do it from a chair with a cellphone. Any Ideas?
The best approach for determining when baked goods are done is to put a thermocouple probe in the item being baked. The basic idea is bake until the probe reports that the item has reached its terminal temperature. Different baked goods may have different terminal temperatures. Bread, as one example, may be done at around 190F. This can be varied based on personal taste and experience.
Thermocouple probes and cables are available that can provide good service life at bake oven temperatures. The probe cable, which is quite thin, can be routed out the oven door and to an electronic device which can read out the probe temperature.
A combination sensor would likely be best.
Even though water in a baked good would boil at 100C (212F) it will still require a certain amount of time for the bulk material to reach a fixed moisture level. Even below the boiling point water still evaporates it just takes more time. So temperature alone would not be an adequate test here.
For most bake goods there should be an ideal temperature and an ideal moisture content that indicates an ideal fully baked state. Matching up a high temperature thermocouple with a high temperature moisture probe should do the trick.
While there are many common high temperature thermocouples available a high temperature moisture probe is not, (as far as I've seen) especially one that might be safe for food use. A non-contact dielectric transmission-line probe can make a moisture detector. If such a probe were coated in high temperature plastic, or Teflon(TM) this might just fit the bill.
I assume the concept could be tested using a thermocouple matched up with a commercially available moisture probe like this one, see:
(Though this one is not suited for high temperature, but it does list an application example as moisture content of bulk foods).
Another solution that would work is to have a small diameter metal tube which draws a tiny amount of air out, as it cools before reaching the detector you could use both a thermocoupler on both ends of the tube and a humidity meter to determine the current moister content inside item baking.
I have seen a few commercial units that does exactly this (measures dry bulb and dew point also).
There are a number of high temperature thermometers available. For your needs, I would recommend one sold by MONNIT (about $250 sensor and gateway) on eBay. This is a wireless system. If you want a cheaper method, there are those with a probe and a digital display for around $10. Temperature range -80 to 300 C. Good luck.