How can you tell if it's a bot vs someone who's just a bad player?

Legit question, I reported someone this morning for repeatedly failing to heal even after specifically being called out. She would leave us when we needed healing, repeatedly. The other healer died multiple times trying to save us. We were up at first, but due to that one healer never healing, we ended up losing. Checked party list and she was a Flame (we were Mael) so I suspected she was throwing it for them but I'd like to know how to tell if it's a bot.