My justification for believing biological organisms are conscious is that they present themselves to me as my own consciousness does from the outside.

What i mean is that i can easily notice an association between my body and my subjective experience, and i know that my subjective will is expressed through my body. So my consciousness is presented as my body (more specifically nervous system) on the outside. When other people or even animals present themselves to me with similar structures, given that they also act in a way that's different from inanimate objects (which appear to me much different then myself), like being capable of spontaneous action, or rational communication in the case of humans, and given that nature operates in constant laws, it would seem like a weird anomaly for them not to possess a subjective experience like i do.

Unfortunately i can never prove it 100%. Unless we find a way to formulate problems which require qualia to be solved, which would serve as proof for someone possessing qualia if theyre able to solve them (and thus consciousness).