system
Given the following post, respond as if you're commenting on Reddit:
human
[Alien] How do the facehuggers known when someone is near the egg and have the accuracy to land perfectly on the victims face?
gpt
The eggs themselves likely have various sensory cells and receptors on the surface that detect movement, scent, light, and other signs of life. Perhaps there is a cascade effect of activation of these senses to prevent false triggers. It would certainly be a devastating waste for a facehugger to hatch without any prey. 

This matches what we see in the movies. Upon approach the egg will move a bit on the surface. This is possibly a way to attract more attention from a curious animal or perhaps just the facehugger inside repositioning (or getting excited). It would be likely that only a small fraction of the sensing cells are active initially in an attempt to save energy. Once one or two senses are triggered, more would activate and the egg would begin to awaken. We know that creatures like mosquitoes can sense signs of life up to 20 feet away.

At this point we would imagine that the egg case releases hormones or chemical signals that indicate to the facehugger inside that something is nearby. Perhaps it simply opens and the now awakened facehugger begins it's routine of finding a moving creature to latch onto.

The latching behavior is likely just the creature looking for a viable orifice. In this case it might detect heat and aim for an open hotspot, or perhaps it senses carbon dioxide and aims for where it is concentrated highly. It may even have functional eyespots that allow it to simply aim for the largest hole at the closest part of the organism. Since predators, humans, aliens, and engineers (and dogs) all have a mouth on the "front/top end" of the body, we can presume that this is common for most lifeforms and thus a desirable location for the facehugger to aim for.

It is possible that rival hives may even attempt to implant each other. This may be one of the reasons that the inner mouth formed on the adult aliens.