True, it's a matter of definitions. Octopuses also use tools, but do they qualify as an intelligent civilization? Probably not (yet). We assume, because we have

True, it's a matter of definitions. Octopuses also use tools, but do they qualify as an intelligent civilization? Probably not (yet). We assume, because we have little reference of what an intelligent human-equivalent entity might be like, that they'd probably build very complex tools over time as we have. This judgement of what constitutes an advanced life form sadly carries a cultural component. If we encountered our own species at the very beginning of its intellectual journey, would we have labeled them a civilization? Would we have described them as having human-equivalent intelligence? It's tricky. But then we move hundreds of thousands of years ahead and suddenly there are monumental buildings and clever machines, suddenly you can meet individuals with astonishing capabilities, and now it's very easy to categorize them. So if our hypothetical space Euroctopus species is in its early intellectual development, we might not be able to correctly attribute their intelligence. There's always the possibility of that development being stagnant too, and again we'd have a difficult time recognizing that. Therefore, the only real chances at discerning an intelligent civilization would be by either directly observing its behavior or by looking at the capabilities of their tools.