When I replaced a 124 evap a few years back, there were two locations which appeared to be leaking. The first was where the large copper inlet/outlet tubing was soldered to the aluminum core. The other was the myriad copper elbows at each end of the small aluminum tubes which comprise the evap core. They too were soldered and showed evidence of leakage.

Don't know that you could detect either of these areas without going all the way into the evap removal anyways.

- JimY