C# - Input Validation for capital letters

I want an input validation for a textbox with capital letters.

For text (or e.g. numbers ...) I use the following way

 public interface IInputValidation
    {
        bool IsValid(string input);
    }

    public class TextValidator : IInputValidation
    {
        public bool IsValid(string input)
        {
            if (input == null || input == "")
            {
                return false;
            }
            else return true;
        }
    }

Can I do a validation in the same way for capital letters? Something like

public class CapitalLetterValidator : IInputValidation
....