Hayward was tracking
the decay signature of vibranium. Based I suppose on the idea that vibranium is rare enough for moving vibranium of that mass to likely be vision.
You don't need a tracking device in the hex to do that as long as atomic particles or possibly certain field effects can get out. Can you do that in real life? probably not due to the low signal but the theory is good enough for sci-fi. Some radioactive materials which are highly unstable can be detected from relatively far away using a
Geiger counter - Wikipedia or
Airborne particulate radioactivity monitoring - Wikipedia.
You can use light to test material composition -
Spectroscopy - Wikipedia.
With this theoretical basis you end up having things like the Enterprise scanning material composition from orbit.
In WandaVision they say that Hayward "found some way to [electronically] see through the hex", so sci-fi requirements have been fulfilled.
Also its still sort of open as to whether hayward has a back-door into the hex.