From the screenplay:
How are they tracking the alien
Ash has constructed motion detectors:
Now if we could only find it.
Ash picks up a portable unit.
I've taken care of that...tracking
device. You set it to search for
a moving object...It hasn't much
range but when you get within a
certain distance it starts beeping.
Ripley takes the tracker from Ash's hand.
What's it key on.
Micro changes in air density.
Keep it pointed ahead of you.
How do they know what happened to Kane
I'm not sure if you mean Kane, or one of the other characters. Kane is the one who has the facehugger attached to him. His fate is witnessed by the rest of the crew when
the alien bursts out of his chest while he is seated in the mess hall with them all.
UPDATE: How do they know what happened to Brett
Brett, Parker, Ripley and Dallas try and capture the alien using a net, but end up startling the ship's cat Jonesy. Concerned that the cat wandering around might confuse their motion tracker, Brett heads after the cat to catch it. Then, according to the screenplay..
Here kitty...Come on Jones.
Brett reaches for Jones.
An arm reaches for Brett.
Now seven feet tall.
Hanging from the undercarriage strut in reverse position.
Grabs Brett and swings up into darkness.
To no avail...
In the doorway Ripley and Parker.
They witness the horror.
In the scene as actually shot (from the director's cut), Ripley and Parker arrive moments after the alien and Brett have disappeared but it's clear to them that the alien has taken Brett.