How do i find out the targeting ability on a weapon?
Wiki knows some but how did they find out?

equip your weapon as primary
target roid or cargo crate
center on it
turn slowly till the targetting thingy changes color. the angle at which it changes is the max autoaim :P

The values in the wiki are mostly copied from the ingame description though.

In order:

Rockets (none)
Railguns
Everything else
Gauss
AGT
Homing missiles

the "everything else" has small variations, but nothing significant. there's not much you need to find out, really...

Gauss should be paired with Plasma Devestators.

In regards to the "everything else", sometimes the variants of weapons have better autoaim--for example, the auto aim on the Axia Accelerated Positron Blaster is better than on the standard, and the Hive Queen Positron has better autoaim than the Axia one. Likewise, the Law Enforcement Neutron Blaster (lawgun for short) has slightly better autoaim than the other Neutron Blaster variants.

the military likes sneaking little extras into their things

Shoot it!

all posi versions have exactly the same autoaim :P