In this particular case I knew because we're all from the same state. The Brahmins from the state wear a sacred thread, and on occasions will smear ash on their foreheads in a particular pattern. One would wear the thread visibly outside his shirt. You can also know based on the last name, skin tone (very light compared to other South indians) and office gossip fills in the rest. One of the VPs mentioned his caste in an after hours drinks party once, I was amused but not shocked.