Zordon knew Tommy was the Green Ranger because the Command Center’s computer identified the Green Ranger’s interspacial biovibrations and matched them to Tommy after he was trapped in the force field. In the episode, Alpha then shows the result on the viewing globe, which is why Zordon and the others realize Tommy’s identity so quickly.
What the scene suggests
The show gives a fairly direct explanation: once the Green Ranger was contained, the system had enough data to run a match. That means Zordon did not “guess” it from intuition alone; he relied on the Command Center’s technology.
Why fans still debate it
Some viewers think it feels a little abrupt, because Zordon seems certain almost immediately. Others explain it as a plot convenience, or as a result of Zordon’s unusual awareness and the computer’s scan working fast once Tommy was exposed.
In plain terms
- Tommy was trapped long enough for the computer to scan him.
- The scan matched his energy signature to the Green Ranger.
- Zordon then accepted the computer’s identification and told the others.
So the simplest answer is: the Command Center’s computer figured it out, and Zordon trusted the match.
TL;DR: Zordon knew because the Command Center scanned the Green Ranger’s energy signature and matched it to Tommy Oliver.