Interestingly, the original script reads slightly differently
[DREDD is in hot pursuit of a vehicle.]
[The pursuit takes place in the tunnels beneath and through the
megablocks, and the open stretches of and fly-overs between.]
DREDD: Dredd to Control. In pursuit of vehicle, sector thirteen, moving west up Wagner Drive. Driving erratic. Suspect driver is
under influence of narcotics.
CONTROL (over radio): Copy, Dredd.
As opposed to what we see in the film
DREDD: Dredd to Control. In pursuit of vehicle, sector thirteen, Driving erratic. Suspects are
under the influence of narcotics.
So the short answer is that Dredd suspects that their erratic driving is the result of drugs. Obviously he can't know for certain, but I'd guess with 30+ years of street experience and countless hours as a motor cop, he'd be pretty confident in his ability to tell the difference between a merely reckless driver and one that's driving under the influence.