Just the way it moves compared to the other dots. They've been doing this type of stuff since like 3rd century BC. Not as precise, but the idea is the same.
It's still mind blowing that they can do that.
yeah i know..but...all of those dots are in different places from the original capture. how you know that one dot is the wonder child planet of our solar system fam? and then to follow up on it with science to be proven correct...that's the stuff that melts my head