I'm going to have to challenge this question a bit, or at least most of the other answers.
When people ask these kinds of things, we are automatically assuming the alien technology and cultures are paralleling our own, in that they see in what we call the visible spectrum, they use what we call radio frequencies, that they form cities, they live above ground and not in the water or in tunnels, breathe oxygen, aren't deathly allergic to nitrogen, are also deathly "allergic" to high levels of radiation, and so much more.
Granted, there will probably be a high level of this parallelism, but it's not necessarily 100% true.
Even if they are looking for some of these things, will they be able to understand them? All of the radio and TV broadcasts are encoded in a way that we understand (or at least our engineers). That doesn't mean an alien race will be able to figure out that these are radio and TV broadcasts, and that they have some meaning to it. If they know to look for this type of radiation, all they'll likely see if a bunch of noise. They won't even be able to likely pinpoint a source more closely than a continent, until they get relatively close. They might be able to determine the general location, since the signal appears and disappears according to the rotation of the earth, but that's about it. What that point is, I don't know, since I'm not a radio frequency buff.
Also, our definitions of sentient life might be significantly different. They might be a hive mind and decide that anything different isn't sentient. They might decide that anything living on the land surface can't be sentient, by their definition. Maybe a dolphin is sentient and anything that enslaves them and makes them jump through hoops for entertainment can't be sentient. Or anything that breathes mere oxygen can't sentient due to not having the right metabolism to have the brainpower to be sentient. There's probably thousands of differing reasons why an alien species wouldn't consider us sentient. I mean, what sentient being would destroy their own environment with trash and chemicals when they can't even vacate the world before it destroys them back?
Then there's technology itself. Maybe they don't use it as we do. Maybe because we use technology, instead of pure brainpower to reshape reality around ourselves, we aren't sentient. Maybe they've harnessed protons for their power and communication, and our electron emissions are not even noticed? Maybe they communicate only in light/color or pheromones, instead of aurally.
As humans that are fairly open about how we communicate and use technology, we like to think that every other species would work like this. Due to them being aliens, as in "not from this world", they are by definition not going to be us and highly likely they won't think like us, regardless what Star Trek, Star Wars, or even FarScape show us. They are far more likely to be something like Predator or Aliens, even if they aren't hostile. We don't/can't communicate with them, only assume some basic thought patterns from their behaviors and decor. They may be so alien that what they consider to be a handshake or the most polite greeting is considered to be horrifying and disgusting to us.
What I'm trying to say is that we can't rely on technological nuances for a species to recognize us as sentient. They may realize technology of our's exists and therefore realize we exist because of it, but not that we are sentient because we have technology.
An alien culture might only recognize individuals who are kind and helping others to be sentient, while those who are greedy and self-serving or violent to not be sentient. Maybe they consider your pet to be sentient, since it's loving and kind. (Ok, so cats are probably not going to count here. :-) )
Again, what I'm trying to say here is that we might get judged to be sentient based on how we act, instead of what we know. Is a gorilla sentient because it uses a stick to break open a coconut to get to the milk and flesh inside, or is it sentient because it learned sign language, or is it sentient because it prevented a predator from killing a defenseless baby? What are the requirements for humans to believe another species is sentient? How homo-centric is that?
I realize I'm probably going way beyond that the OP is asking, since they are asking more as a story or plot point, but it might be interesting to explore some of the questions I've asked, rather than rehashing the same old plots of "We've detected life on this planet 1 parsec that way, and it has a colony, so they must be humanoid sentient creatures like us." Maybe what I've asked is exactly what the OP wants or what the OP already has questions about. Maybe I'm just showing how OCD I am. If the latter is true, sorry about that. Truly.