Sometimes the simplest things can be the most helpful and homebrew software like controller test tools often fall into that category. Developer Hammer☆Keyboard☆Studios has just released exactly that for the Game Boy Advance and while I think the tool is helpful overall, there’s one specific test I think will be its main use case: Testing the triggers! I’ve used many GBASP’s with dying triggers and software like this is an easy way to test if they’re good, or if you’ll need to replace them.
Check it out yourself here: https://www.romhacking.net/homebrew/142/
Also, Pinobatch has been maintaining a ported version of the 240p test suite on the GB, GBC and GBA. They’ve even integrated MDFourier analysis, to help test and improve audio accuracy on mods! If you need a full set of tools to test your classic Nintendo handhelds, both of these pieces of software have you covered!: https://github.com/pinobatch/240p-test-mini