Its 100% true in my experience. You can identify caste of most Indians based on their last names. This is one of the reasons some Indians do not use last name at all. (e.g. from Tamil Nadu or from Northern states of India). At work you can't hide your last name, so other Indian co-workers can identify your caste with trivial effort. However, I try to hide my last name in any type of social (non-work) related conversations and try to avoid any groups where group members are really curious about my last name.