Usually by end of first year. I'll tell with respect to IISc. CSA where there are 3 broad areas.
1. Theoretical - where work is on solving unsolved problems, finding better algorithms etc. and most people stay in academia. Related subjects include algorithms, TOC, graph theory etc.
2. System - usually considered tough as system level coding is involved. But jobs are always there and competition is less. Subjects include Databases, CO & A, Compilers, Programming Languages etc.
3. Intelligence- more into mathematics and is currently hot. Needed by Software giants and they pay the most- Amazon, Walmart, IBM IRL etc. Linear Algebra, Probability theory are the basics.
Usually one can decide his interest (for M.Tech.) based on how the first year goes where most subjects are compulsory.