How to check if a StringBuilder is empty?

If you look at the documentation of StringBuilder it has only 4 properties. One of them is Length.

The length of a StringBuilder object is defined by its number of Char objects.

You can use the Length property:

Gets or sets the length of the current StringBuilder object.

StringBuilder sb = new StringBuilder();

if (sb.Length != 0)
{
    // you have found some difference
}

Another possibility would be to treat it as a string by using the String.IsNullOrEmpty method and condense the builder to a string using the ToString method. You can even grab the resulting string and assign it to a variable which you would use if you have found some differences:

string difference = ""; 

if (!String.IsNullOrEmpty(difference = sb.ToString()))
{
    Console.WriteLine(difference);      
}

use the StringBuilder.Length Property, here the doc

if (mySB.Length > 0)
{
     Console.WriteLine("Bang! is not empty!"); 
}