Actually, it's not hard at all to tell that this is a bot.
After running into the 3x Anubisath Idol bot team repeatedly last night and watching it make the exact same choices and exact same timing in more than 8+ battles, I decided to do a bit of research.
This is in fact a bot and I found the bot program that runs it, and the "Pet Battle addon" for this bot program. I'll not publish the names or links here, but it's fairly easy to find if you know where to look or just do a bit of searching.
This bot program is being used by botters for leveling characters through the XP gained from pet battles. Apparently a lot of botters have been banned for the traditional botting methods recently (farming herbs/ores and battleground PvP) so Pet Battles are a new method they are using to level up characters on fresh accounts, or to get their alts to max level. They are currently not being reported and probably have much less risk of being banned because of the way pet battles are anonymized.
The 3x Anub bot team is easy to spot, it makes the following moves EVERY time without deviation:
1. It does not pick a pet to start, rather it lets the time expire so one of the 3 Anubs is picked by default.
2. It uses Rupture, Sandstorm, then spams Crush until Rupture/Sandstorm are off cooldown.
3. When any of it's pets are below 25% health it will swap them out. Even if you get all 3 pets below 25% it will never attack you again but just keep swapping pets every turn until you kill them all and win.
The reason for #3 is that the bot is coded to swap out any pet below 25% health. The developer of the bot program thinks that this is what any "normal" person would do. I have found it makes this team very easy to beat when you know it is coming, so I was happy to farm this bot for wins
I suspect that there are actually many more bot teams which are less obvious to spot at first because they are using different pets and rotation. It will be interesting to see if blizz starts banning these or if we end up with as many bots in pet battles as there used to be in battlegrounds.