Ok, so we tend to suspect suffering wherever correlates of suffering are present, whether they might be
- behaviour correlated with suffering
- situations correlated with suffering
- physiological reactions correlated with suffering
- correlates to physical correlates of suffering (such as certain patterns or substances in nervous systems that correlate with suffering)
- other things that might correlate with suffering
But of course correlation doesn’t imply causation, so we have to be careful, and perhaps even scientific about how we connect the correlates of suffering to our subjective estimates of the suffering present. For example, it we would suppose that weapons correlate with suffering, but we are also aware that weapons mostly only cause most suffering when they are actually used against persons. So, the context of an association with suffering plays an important role.
The more knowledge we have, the better we are at estimating the likelihood of suffering. A naive person might assume that a fakir suffers a lot when sitting on a board full of pointy nails, but seems to be very good at suppressing suffering. The more than person learns about the reasons why it’s not too painful at all to sit on such a board, the less likely is he to assume that the situation should be interpreted as suppression of actual suffering.
Conversely, we might learn about signs of distress in other animal species and rightfully assume that they indicate the presence of suffering, even if those signs of distress are unlike those of humans.
The more we know, the more accurately we can assess the presence of suffering (or other feelings for that matter).