The proper way to check preheat is with:

Using a temperature indicating crayon is the preferred method for checking preheat because it provides a direct and measurable indication of temperature. These crayons are designed to melt at specific temperatures, allowing you to verify whether the oven has reached the desired preheat level. This method is reliable and ensures that you achieve consistency in your cooking or baking tasks, as many recipes require precise temperatures for optimal results.

The use of a wet finger, while it might provide a quick visual cue, does not offer an accurate reading of the temperature and can be subjective. An indoor-outdoor thermometer, while useful for monitoring ambient temperatures, is not designed for checking the surface temperature of an oven, making it less effective for ensuring proper preheat conditions. Therefore, the temperature indicating crayon is the most accurate and reliable method for ensuring your oven is adequately preheated.