system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] How do Genestealers know who to infect and who to slash to ribbons?

Also, do they instinctively know that the Terminators aboard their Space Hulk are no good for reproduction?

**Edit** - After thinking about the question on and off overnight (as well as rereading relevant portions of some Ciaphas Cain books), I have a new working theory: Genestealers will kill everyone in a group and then only incapacitate one or two "lone survivors" and subsequently infect them and eventually let them go to spread their now-tainted genes.  

The survivors will be disoriented, as "The implant messes with brain chemistry, so the host remains unaware of being infected.  All they recall is a confused impression of fighting and assume they've escaped."  This is chalked up to PTSD and the hapless dupe goes on to spread genestealer taint.

As to the question of Terminators, it's possible that 'stealers are able to tell if their implant "takes".  They shred *most* of the Terminator squad, incapacitate the last one, give him the implant, Astartes super immune system kills off the infection, Genestealers know the implant didn't work and dispatch the helpless space marine.

Good working theory?  Anyone see any holes?
gpt
Genestealers in particular aren't stupid, they know what is prey and what will give them a fight (i.e. They know terminators are to tough to take down non lethally). In general though, the Hive Mind has no doubt encountered Space Marines before and has worked out, and passed on this information, that SM's don't reproduce. 

It's possible that an old group of genestealers that hasn't been in contact with the hive mind and hasn't encountered SM's before might give infecting a go.