Would we detect a spacecraft before it landed on Earth? We have satellites and telescopes looking outward and inward to the earth. Would any of these detect an extraterrestrial spacecraft before it landed, or in other words, would we know a spacecraft was approaching before it did (assuming its going at some reasonably detectable speed in our local area)?
Let's assume it is Saturn V like wrt detectability.
Some thoughts about Earth-based detection systems:

We are looking at asteroids using radar: For instance the former Arecibo telescope could actively send out radio waves and calculate object position and size. However, we might be able to detect an incoming spaceship using this technology, assuming that the object approaches Earth within the radio beam, if we we are currently studying another object. In other words: Detecting an incoming object would be purely chance.

An all sky camera network for meteorite detection is another outward-looking system worth mentioning, but that would be very very briefly before landing of a hypothetical extraterrestrial space ship. I am sure you are not after this option since we would be seeing the object already with naked eyes if it would not be incoming at a very remote location. However, this option is to my knowledge working 365/24/7 and covers large portions of the sky, I believe.

Edit suggested by uhoh: There are also initiatives like NASA's Space Fence or ESA's Space Surveillance and Tracking (SST) segment monitoring space debris within the geospace which could possibly detect other objects as well, see also https://space.stackexchange.com/q/45144/12102


Concerning satellite-based observations:
If the objects emits significant radiation e.g. in parts of the spectrum of communication satellites, we might detect anomalities or malfunctions in one or more satellire and by chance figure out the extraterrestrial source of the distortion. This would of course only hold for slow approaching objects.