Wiki

/// <summary>
/// checks all fields if they are required, and if so, check if the item has a value for the field
/// </summary>
/// <param name="item"></param>
/// <returns></returns>
public static bool RequiredFieldsSet(this SPListItem item)
{
    foreach (SPField field in item.Fields)
    {
        if (field.Required == false) continue;
        if (field.GetValidatedString(item[field.Id]).IsNullOrEmpty())
        {
            // field has no value
            return true;
        }
    }
    return true;
}