Moisture Detector for Ceramic Tile

Is the subject tile in hand, or on the wall/floor?

If in hand, you might weigh a representative group of tiles, rest these in an oven (for example) at 200ºF or so for a while, then weigh again. Might take a while for any water to migrate out.

If installed, I cannot think of any NDT method to get a reasonably believable result.

If this is truly a quest you are pursuing, perhaps contact the Tile Council of North America and see what they have to say.

https://tcnatile.com/

There are ISO specs for Tiles ISO13006 10545/98
As Doorman said, to correctly control the moisture content would be by weight but if the tiles are already laid, that would be difficult to do.
The chances are more likely to lie in the substrate on which the tiles are mounted and the preparation work carried out before the tiles were laid.
While most tiles are virtually imperverious to water absorbtion the Grout between them can also let water through.
Its an interesting theme so why are you asking?
Info on various sensors monitoring moisture content in walls which are tiled - Suchen

I recently had a washing machine that overflowed due to a defective level control switch. I have ceramic tile on top of plywood substrate and the company performing the mitigation and remedy claim their instruments can read through ceramic tile to the substrate below.

The insulation under the floor was removed and moisture was measured and a dehumidifier and fans were placed under the house. I have no problem with this aspect of their job, but the tile moisture reading is suspect. There were 3 fans and 1 dehumidifier on the tile inside; Very noisy and inconvenient to say the least.

They justify the fans by the moisture content of the tile. I am not sure of the accuracy of their tools.

The only way Absolute Moisture can be measured is a dry sample weight vs a unknown sample of exactly same material. I worked in an ASTM for a while and they had a scale( ASTM Class 000) that could weigh your signature. I calibrated non contact moisture meters that used NIR and IR to measure material moisture even if a curtain of steam was in between the UUT and the sensor. I had a standard tile used to define 20% and another for 75 for calibration reference.

IMHO:

I think the company may be adding unnecessary time to the job. ($$)

Thanks in advance for all help on this matter.

Oh dear, accidents happen unfortunately. As you say they removed the insulation under the floor I am assuming your house has a gap between the earth and the floor of your house.
Having a number of fans running in the tiled room will not be very effective as they are trying to dry moisture out of a ceramic tile which is not very porous and most of the water will have soaked through the Grout..
It would be far more effective to remove the moisture from below by sealing off the area open to the underneath and blowing warm air through it, but I have a question, are they just using fans circulating the air or using Heaters with fans attached ??
Two friends of mine had water damage in their walls due to a tenant above them having a defective dishwasher, they had heaters running for weeks before the plaster dried out. The noise was deafening and the electricity meter was turning like crazy.
Where you will need to very careful is if the water has gone into the plywood it will separate and swell. It maybe cheaper in the long run to have the floor tiles removed and lay a complete new floor.
Again it depends on how much water went into the floor and where ? Is it possible that water has gone into the wall itself and that the wall is also wet ??

I'm only guessing, but I'd think that there would be a direct, possibly linear, relationship between the moisture content of tile and its ability to act as a plate of a capacitor. Perhaps two flat plates pressed against the surface of the tile, so the tile acts as a third plate.

This would only work if the surface of the tile were completely dry.

And no substrate (plywood floor) inbetween ! and you would need a new tile to use as a reference factor.
One could use a tool to measure the Moisture reading on top the tile and one from underneath the plywood floor, there should be a massive difference in the values

Perhaps a thermal camera can reveal a temperature difference between areas that are wet or dry underneath.