Keith: you're talking about the attack numbers, but MayA was talking about threat. For attack numbers, those can be validly seen by the player, but for threat that's something that is not known to the human players because the AI is out in the fog of war somewhere unknown.
MayA: the only way to see the information you're talking about is to wait for them to attack you, or to put scouts on the AI planets near your own, and eyeball it. Given that these are hidden troop movements of the AI, you can't find out what they do until they tip their hand. Your best bet is often to make sure that you have reasonably few ingress points, so you don't have so much uncovered flanks.
But, just having a mark I scout sitting on each AI world adjacent to yours can also give you a visual indicator of where ships are moving around, as when those numbers start counting up and you have high threat, you know that at least SOME of the AI ships are there.