There are a number of signs and signifiers. Firstly, I have been searching extensively for signs of alien life on this planet, and I have found none - and believe me, I know the signs. Second, I have see the schematics for the porter device itself, and while much of the technology is...

[a pause. it really, really irks him that this is the case]

... unfamiliar to me, there are certain design decisions, ways of constructing it that bear the stamp of mankind. Finally, when she makes her little gloating posts to the network I've made it a habit to trace her signal as far as I can, try to see what's behind her code. If she is truly an AI - which seems to be the case - the snatches I've seen bear the same underlying logic. It is much like language - you can tell that a man is from a particular country by his dialect and his mannerisms, even if he hides it particularly well, there are generally certain tells. So it is with this one - she is advanced to a point that to a layman it might be difficult, but I can see the signs.

While it's possible she was still designed by some other force, I believe that the evidence - a combination of a lack of anything else in this universe to have made her, and those little signals and signs leads me to that conclusion.