Not sure what roach timing you're referring to.
There are many possible timings, on one base, two bases, three-base roach max, etc.
Also, you haven't specified if you're opening gateway first or forge FE.
Given that the metagame right now is for gateway openings, you should scout on 9 or 13. If you see he's on a single base you drop 2 extra gateways and make a sentry and stalkers to handle the pressure.
If it's a 2base roach all-in then you'll know by the empty third base (can be scouted with a probe, MSC, hallucinated phx, etc.
Basically you just identify that the zerg isn't droning and make extra gates or boost out immortals to handle the problem.
If you forge FE it's harder ... you need to make a bunch of cannons (like 4) and boost out a few sentries to keep them from getting into your natural. May need to pull probes to keep the roaches from killing your cannons.