Well, you can do like this.
First : check input value is Decimal write(there many ways to determine it)
Second: input string contains fractional part of number.
Third : the length fractional part of numberof is more than 2.
See below.
string[] strSplits = textbox1.Text.Split(new char[] {'.'});
if (strSplits.Length > 2)
{
}
else if (String.IsNullOrEmpty(strSplits[1]))
{
}
else if (strSplits[1].Length > 2)
{
}