A: 2 observers. If you can get one observer near his forces or the front of his base, you should know if he send medi's out.
B: 6 warpgates. This will dampen the effect of the drop, especially if you have khadarin amulet templars.
C: A single photon at your mineral line. This will allow you to hold off the drop units long enough to get your probes out of the danger zone.
D: Phoenixes to kill the drops before they drop.
Honestly though, most of my PvT's end before they get to the point where they're comfortable dropping. Early HT into an agressive push has yet to lose for me. I need to start playing T's with better micro.