If the moon had life and it progressed at the same rate as life on earth, including key points of technological discovery. When would we, on earth, have first detected them?
Technological discoveries I mention are things like... fire, Stone Age, Bronze Age, industrial revolution, etc... first telescopes, electricity, radio, flight, etc...
(Obviously ignoring the fact that the moon is lifeless with no atmosphere)
There is no doubt that by now we would know they are there. We would receive the radio waves they emit. We have satellites that would clearly see their cities and road infrastructure.
Before that there could be electric lights lighting up at nighttime that could be detectable by powerful telescopes.
But in the past we wouldn't know they were there. So there is a point when the first discovery was made.
What point in technological advancement would be the earliest point where we would have noticed them without knowing they were there and realised that they were a civilisation.