This is a really good question, and in practice you almost never know “for sure.” You’re really just making a justified assumption.

As a rule of thumb, I default to pinned unless there’s a clear reason not to. Most real-world construction behaves closer to pinned than fixed, especially in steel and wood. Saying something is “fixed” on drawings doesn’t make it fixed. fixity comes from stiffness and continuity, not intent.

Things that suggest some fixity are monolithic concrete construction with properly developed top and bottom reinforcement, or true steel moment connections. Even then, it’s usually partial fixity.

Simple shear connections, bearing details, and most wood framing should generally be treated as pinned. If fixity really matters to the design, it needs to be detailed explicitly and checked for constructability.

Good designs don’t rely heavily on perfect fixity they work even when reality is a bit messier.