To check if a stringbuilder is empty, use the Length property.

i.e

private readonly StringBuilder builder = new StringBuilder();

public void AddSomeString(string someString)
{
  if (builder.Length > 0) builder.Append(", ");
  
  builder.Append(someString);
}

public string CommaSeparatedString
{
  get
  {
    return builder.ToString();
  }
}