I have a model classes that has a description property with a data annotation attribute of StringLength and length is set to 100 characters. When this property is more than 100
You could always check the attribute value using reflection, though that approach is not the best if you can get around it - it's not pretty:
var attribute = typeof(ModelClass).GetProperties()
.Where(p => p.Name == "Description")
.Single()
.GetCustomAttributes(typeof(StringLengthAttribute), true)
.Single() as StringLengthAttribute;
Console.WriteLine("Maximum Length: {0}", attribute.MaximumLength);