Judging whether someone is "intentionally feeding" rests on the word "intentional". Intent is something that is completely internal. The only way know someone's intent is if they communicate it to you. I've seen it happen a few times. Just yesterday someone said in chat "If you guys don't surrender, I'm going to feed." When we didn't surrender, he started dying a lot ... mostly walking straight into the enemies.
Now, if he hadn't said that first, there could be any number of reasons for that to happen ... mouse glitch, lag, GPU glitch, poor skill, taking a bite of cereal while reading Tolstoy. I almost never claim intentional feeding for that reason. The only valid time is if they communicate their intent.