If you can build a spaceship that can effectively travel to another star system you have the exact technology needed to build a fucking telescope when you get there. The very ship you use for the trip likely has a very capable telescope as it would be needed to avoid interstellar debris en route.
The scale and scope of "building a telescope" and "building an interstellar spacecraft" are not just comparable but directly related. I'd also assume such a civilization would have a good handle on diseases, at least have a strong understanding of them.
It turns out that the civilization on Earth that landed on the Moon also had a pretty decent understanding of disease. By the time of the Moon landing many diseases that wracked the world in the 1500s were well under control in the first world. Some were even eliminated.
Telescopes are great but aren't going to tell you much about the exact radar capabilities of a nuclear aircraft carrier, nor what tactics it would use if it did see a craft, nor submarines (one of the incident reports claimed the tictac went underwater). And monitoring Earth is a very general thing, it can even include getting up close with the native wildlife, it isn't restricted as a concept to viewing from a great distance.
The scale and scope of "building a telescope" and "building an interstellar spacecraft" are not just comparable but directly related. I'd also assume such a civilization would have a good handle on diseases, at least have a strong understanding of them.
It turns out that the civilization on Earth that landed on the Moon also had a pretty decent understanding of disease. By the time of the Moon landing many diseases that wracked the world in the 1500s were well under control in the first world. Some were even eliminated.