We almost certainly wouldn't see the alien ships until they were in orbit, particularly if they approached from one of the solar polls rather than in the ecliptic plane (the geometric plane that contains the planets and asteroids).
We can get radar observations from objects as far away as Saturn, *but we have to already know they're there* to observe them with radar astronomy, and they have to be quite large -- 100s of km across. Even as far away as the moon an object would have to be a km across to be caught on radar. So we don't send radar signals willy-nilly into space unless we know the object is already there. The way we detect near-Earth objects like asteroids is optically, looking for "stars" that move across photos taken in succession. But this might not detect the approaching fleet at all, even if it were approaching along the ecliptic; and if it did it's likely that we wouldn't notice for days. The system isn't designed to detect fast spacecraft maneuvering toward Earth; it's designed to detect rocks more or less traveling along with us that wander into our gravity well.
Of course all this depends on your assumptions about the ships. If the ships were as big as the Moon, we'd notice them from a few AU away. If they emitted exhaust plumes that were bright as Jupiter, we might even see them with our naked eyes well before they reached orbit. But if they're only a few km across and not fantastically bright, chances are we wouldn't notice them until they showed up on our orbital debris tracking system. Even then we wouldn't necessarily notice right away. The system isn't a real-time early-warning system. We'd probably still be chasing down the "glitch" in our systems when the first aliens landed.
Now I wanted to answer your question, because it raises and almighty rant in me that I just have to get out: is it too much to ask that writers of "science fiction" have a *little* science knowledge and a more-than-room-temperature IQ? For Pete's sake the energy in life forms (at least Earth ones) is solar radiation converted into chemical bonds. The notion that a spacefaring species would have to transport those chemical bonds across insterstellar distances for its *energy* needs is preposterous. The notion that this would net them any usable energy is nearly as preposterous. Would you send a log into orbit to fire a boiler?
It's not just second string popular sci-fi that has this problem. Both the J.J. Abrams Star Trek movies show a complete lack of thinking about the geometry of space. In the last movie the Enterprise is chased across interstellar space only to be stopped 240,000 km from Earth, and they're *right by the Moon*. Yes, 240,000 km is roughly the radius of the Moon's orbit, but the chance you approached from some random direction and happened to end up right by the moon is minuscule, even if you're at the right distance. And then when they lose power they *instantly* fall *straight down* into the Earth's atmosphere.
Yeah, I understand it's the storytelling that counts, but it matters if the scenario is just plain stupid.