Extendscript: Possible to check if layer is used in ANY effect?

As Mathias already stated, the layer dropdown controls are populated in the same order as the timeline itself. That would be pretty much the only way to verify if a layer is referenced anywhere. Mathias also makes some good points about inert layers that are seemingly not being used anywhere, but may be referenced in expressions. To add to that, it could even go so far as the reference actually being constructed inside the expression itself, in which case you don't even have a clean string to check against. I used to do such stuff in some of my old data graphics projects way back when. So technically there is never a clearcut 100% surefire way of being sure if a layer is actually not being used somewhere.

Mylenium

I think a layer dropdown effect control always lists all the layers of the current composition and always lists them in the same order as they occur in the comp. So if you know that the second value of the dropdown is chosen, you know that it must be the second layer of the comp, right?

Another thing to keep in mind:

Layers might also be accessed by expressions. The source text of a layer could, for example, be linked to the name of another layer, and removing the layer will break the expression. Finding references to layers in expressions is probably nearly impossible to do properly.

If an expression contains
layer("my layer")

for example, you know that it uses a layer with name "my layer". But depending on what other code is around that statement, it could also be "my layer" of another comp. And if your expression contains something like

layer(i)

you would need to inspect the expression in more detail to figure out which value the variable i might have...