[ASM & Hex] FireRed - Checking which Pokemon was caught

I am trying to work out a check on whether a specific Pokemon has been caught.

The only post remotely related I could find was:
https://www.pokecommunity.com/showthread.php?t=249530

The way I understand it, the post explains how to pass parameter via R1 and R0 to the ASM routine to check whether a Pokemon is caught/seen.

But how do I get the routine's result back?

The plan is to write my own Thumb routine, that takes in parameter from scripts, call the original game check, and return check result to script variables.

calling function 0x104AB0 with species ID in r0, and r1 as an index (0: Check seen, 1: Check Caught, 2: Set Seen, 3: Set Caught) will return the result to r0 (0x1 caught/seen, 0x0 otherwise, I assume)