[chiralSPO (OP)]

 Imagine filming a viscous fluid, like honey. Sometimes time lapses of veery viscous fluids moving can look like normal speed of less viscous liquids, and increased playback speed of less viscous liquids can look like normal speed of more viscous ones. But it's not perfect. What kinds of motions or observations would allow one to unambiguously tell whether the honey was hotter (less viscous), or the playback speed was increased?

i.e. if there is a droplet free falling that is obvious, because the acceleration is constant.

[alancalverd]

Look at the velocity profile from the surface to the line of maximum flow. A nonviscous liquid will have a very steep initial gradient from the boundary layer to rapid flow, whilst a viscous liquid will have a fairly constant gradient from edge to centre (of a tube) or surface to leading edge (of a liquid sliding over an inclined plane). A viscous liquid in contact with a single plane will have a distinct leading bulge.

[evan_au]

At over 80 years, this is claimed to be the world's longest-running scientific experiment: The pitch-drop experiment.
It is looking at viscous flow.