You can get field type with display name.

private string GetFieldTypeForCaml(SPList list, string displayName)
{
string sFieldType = string.Empty;
for (int i = 0; i < list.Fields.Count; i++)
{
if (list.Fields[i].Title.Trim() == displayName.Trim())
{
sFieldType = list.Fields[i].Type.ToString();
break;
}
}
return sFieldType;
}
Enjoy~! DDIBA 