There are a few ways to tell when an oven is preheated without opening the door:
Visual indicators: Some ovens have visual indicators, such as a light or an LED display, that illuminates or changes color when the oven reaches the set temperature. Check your oven's manual to see if it has this feature.
Audible signals: Certain ovens produce a sound or beep when they reach the desired preheated temperature. Again, consult your oven's manual to see if it offers this function.
Thermometer: You can use an oven-safe thermometer placed inside the oven to monitor the temperature. Place the thermometer on the center rack and look through the oven window to see when it reaches the desired temperature.
Thermal gun/infrared thermometer: If you have access to a thermal gun or infrared thermometer, you can point it at the oven's interior through the window to measure the temperature without opening the door.
Preheating time estimate: Most ovens take around 10 to 15 minutes to preheat to a standard baking temperature, like 350°F (175°C) or 375°F (190°C). However, preheating time can vary depending on the oven model and the desired temperature, so it's a rough estimate.
Remember that using any temperature-measuring device or opening the oven door while preheating may slightly affect the actual temperature inside the oven, so it's essential to be mindful of that. Additionally, it's a good idea to check your oven's manual for specific instructions on preheating and any unique features it may have.