It's bit tricky-I also make a MMC5 homebrew(it'll take some time to finish) that also use ExRAM.
When you write to ExRAM, CPU(?) mirror $5000-$5FFF to $6000-$6FFFF.
In other words, you can see what you'v written at $6000-$6FFF, but changing those won't do anything.
I discovered while making my Bio Miracle MMC5 hack that many emulators don't support MMC5 ExRAM so well and that atleast some wouldn't let you execute code from it which the real cartridge will.
If you can alternatively use PRG-RAM for development and debugging you could always move to ExRAM later. I forget if that's what I did as it's been so long since I dealt with it.