Why would we assume that Troodon was intelligent for an animal despite never seeing a live one?

The assumption of intelligence in Troodon and some other dinosaurs is based on indirect evidence and comparisons with living animals. Here's why researchers hypothesize about Troodon's intelligence:

Brain-to-Body Ratio: Troodon had a relatively large brain compared to its body size, similar to that of modern birds. In living animals, a larger brain relative to body size is often associated with higher cognitive abilities.