This video explains how to check IVs in 3rg gen games using an emulator and RSEDecode. Although the video is done using Emerald as an example, all 3rd-gen Pokemon games will work. Where I enter "2024550" as the memory address, please replace it with one of the following depending on the game you are using:
Ruby/Sapphire - 30043C4 Fire Red/Leaf Green - 20242E8
NOTE: This is intended as a general "how do I use this?" tutorial only. As I am not the developer of RSEDecode, I cannot answer and technical questions about it regarding bugs or failure to load.