Well, the field surrounding pain/injury is large and nuanced, so not only is that a difficult question to answer but it's also sort of the wrong question. And, firstly, I strongly recommend watching the linked video (if you have time). But I'll try my best here anyway.

Let's start with injury - what constitutes an injury? As in, how should the word "injury" be defined? This is a more complex issue than one might initially assume; not all pain is associated with an insult to tissue, not all insults lead to mechanical damage, insults that do lead to damage do not necessarily lead to pain, etc. So, mostly, injury tends to be defined as anything that leads to a reduction in function/performance in an organism. The ramifications of this definition are important, because things like a compound fracture, delayed-onset muscle soreness, and pain with no (known) underlying cause all meet the definition's criteria.

That last bit is the most important (and why I strongly recommend watching the video), because pain is not an input to the brain, rather it is an output of some function in the brain that takes a large number of inputs other than pain. This is basically the crux of the whole bio-psycho-social model, and why I said the original question is sort of the wrong question regarding injury. So here's the revelatory part, as it relates to the original question: aside from large and obvious traumas like a compound fracture, one cannot conclude that, because they are experiencing pain, physical damage to tissue has taken place. So looking for a heuristic to determine the state of some tissue after experiencing an event that led to pain is not really the right approach from the beginning, as one cannot be confident (and should not assume) that the tissue's state is awry at all.