How? His phone puts him in the general area on multiple occasions, but that's not specific enough.
There's no known surveillance footage that proves this.
No evidence from any of the victims that they were stalked, and finding digital evidence on his devices is hit or miss (as we've learned from plenty of cases on here). He had plenty of time to cover his tracks in the days and weeks following the crime.
So it absolutely could have happened, I'd bet the farm on it happening, but you cannot prove that it happened, as knowing something and proving something are two completely different things.
So I ask you, how can they prove it?
Yes.
IMO the prosecution will likely produce some evidence of planning and premeditation - some of which might loosely be called 'stalking'. Stalking is a red flag pre-cursor that appears in many cases like this. But I would not expect it to be charged as such. Like you, my guess is he pre-selected the property after observation.
On the other hand, you don't want "fake news" entering the trial e.g bogus reports about 'stalking' as that would be prejudicial.
MOO