The search for extraterrestrial life is an exciting one full of uncertainty. I was actually first surprised by the level of effort in this field, for there is such a high probability you may spend years and find nothing. Worse so, not because there are no other advanced communicative civilizations in the milky way, but because we have no definitive way to actually detect them.
The leading way at the moment is to take advantage of the fact that space is mostly hydrogen, and that this frequency has typical very little natural activity. In fact, the whole terrestrial micro range has very little activity. Therefore, we are looking for radio waves and signals that we believe to be unnatural. If one occurs, we assume that it must be a civilization producing it, not necessarily to speak to us but just in general. This is definitely more scientific than I had assumed a search for aliens might look like, and it is quite promising. However, it is not entirely accurate, as seen with the false positives, and relies heavily on a chain of assumptions, and the major assumption that in a tiny knowledge of the universe on Earth, we can derive knowledge of all natural vs unnatural frequencies. From the Drake equation to the frequency analysis, uncertainty seems to be the defining characteristic of this field, but the level of it is decreasing, which is encouraging.