What is the simplest way to validate the controls in UWP?

What do you mean by validating the controls? The data of the controls is validated, instead of the XAML controls. You can do so, such as,

<textbox name="username" />

Suppose you have to "validate" that the user must enter the username to be of length 5-25. You can validate the input using the C# code (or VB.NET if you are using that langauge),

string inputData = username.Text; // Get the text

// Validate
if(inputData.Length < 5 || inputData.Length > 25) {
   // Notify the user about the error
}

Similarly, you can perform other checks on the controls that you are using, you can use their property values and then check them against the values that you want.

Jerry Nixon has written just a post about it, Jerry Nixon on Windows: Let’s Code! Handling validation in your Windows Store app (WinRT-XAML).[^]