Highly polarized debates around fishes’ ability to feel pain concern sentience: do reactions to tissue damage indicate phenomenal consciousness or mere nociception? Yet we lack ‘bullet-proof’ neurological and behavioural markers of sentience that would be sufficient to convince skeptics that fish feel conscious pain. So here, we tackle the various definitions of sentience, and present several paradigms that may help scientists and advocates navigate the existing literature and pursue promising future research. To do this, we identify certain organisms widely held to be non-sentient: plants (P), spines disconnected from brains (S), decerebrate mammals (D), and humans in states of unawareness (U). We review the impressive suite of abilities that these “S.P.U.D. subjects” are capable of, such as: approach and withdrawal; reacting to stimuli with apparent emotion; discriminating between stimuli; showing Pavlovian learning, including trace conditioning; and even learning instrumental responses. This makes the presence of these responses and abilities unconvincing to animal sentience skeptics. In the search for stronger evidence, we propose experiments that might yield more convincing data, as a constructive route forward for researchers. Finally, we address how to balance continuing scientific debate around sentience with welfare protection, discussing the main concerns raised by key players in the fish pain debate as an example, and reviewing how and why precautionary approaches are used in policy. Please note that some of the experiments reviewed in this presentation may be disturbing to audience members, as they involve sometimes highly invasive procedures on animal subjects.