Get the Imperial Assault Campaign module for Vassal from http://www.vassalengine.org/wiki/Module:Star_Wars:_Imperial_Assault
The next Total Solar Eclipse holiday in 2024 in USA? See you there!
Because placing evidence can be seen and not expressly defined secret, it is open information which one was placed by which detective. (Also thematically each evidence is trackable.)
You can use any method you like to track it if you like.
I really don't expect that information to be of much help though, because due to how the evidence files are processed in the end, a detective would want to place at least one low and one high evidence to the evidence file of the guilty hunch. (Alibi tokens and alibi and perjury evidence tokens can also be used to obfuscate who you want convicted and who cleared.)
Knowing what the revealed value is has a markedly larger importance than who placed it.