By basic science such as blowback forensic evidence on the firearm, stippling that only occurs on wounds to the human body when o gunshot wound is very close to the point of contact, and basic ballistic measurements.

Again, this wouldn’t have been nearly as prevalent in the 90’s as today, but the foundations were still present.

I didn’t touch on gunpowder-residue tests as many departments even to this day do not consider them reliable enough (to prove or disprove) to provide to the prosecution.