This page gives you a total list of what you have and what you are missing.
If you go to a summary battle page like https://wow-petguide.com/index.php?m=WailingCritters and if you have an account with a registered character so your collection is held, and you press the Regular or Heroic Mode button, it will list how the pets you have match up with the strats, and what pets you are missing for those specific strats.
I'm late to the party but for anyone experiencing an error with importing your character, make sure to enable the 'share my game data with community developers' option in your Blizzard account privacy settings. That was what worked for me at least.
Wow!! I didn't know about this either, but now that whole API rework makes a lot more sense. I'll add this info to the error message as a potential option. Sadly from my perspective the error code I get from Blizzard is the same whether it's a non-existing character or one protected through this option.
I have no idea why this is happening. The transmition works fine and there are no formatting problems but for some reason the armory is just not giving a result back :-(
I read elsewhere that there are some problems on Blizzards side and for now that's my best explanation as well. Not much I can do unfortunately