Personally when I have hallucinations, they have a sort of grainy or too-bright quality about them that I will only notice either after or if I start to question it.

My philosophy is all my voices and hallucinations are real, it's just my judgement and perception that can be false about them. Mindfulness helps discern the reality of the hallucinations.

This is how I know my visual ones Aren't real 1: the don't change they Always where the same clothes and they dont age 2: they tend to repeat the same words

I have a dog, she's a pit/boxer mix named Rosaline. Personally, I've found that if the animals around me aren't reacting, then it's not real. Humans may not notice something and it leaves you questioning, but animals can just sense stuff. If the man across the road staring at me, unmoving, was real, I know my dog would get into defense position. If she doesn't then it isn't real.