Since a couple of people seem confused, the players are there to purchase tech made by human colonists. The tech was made by exploiting the native alien fauna. The ecosystem of the planet is well documented but the aliens in question are reclusive tool users so the players don't know they are smart. The evil company exploiting them DOES know of their intelligence but chooses to continue capturing and experimenting on them.
Setting: RPG Sci-Fi
My players will be heading to a metal-rich fungal jungle world where many lifeforms incorporate more metal than the lifeforms on our planet. (I know the chemistry of this is mental and unrealistic but I'm not a biologist and those parts of the science are quite soft. I just want to tell a story.)
The planet is a recently settled colony on the near-side of Andromeda. It has been settled by a Medical research company because the planet is a "Super-Earth" (theorised places better for life than earth) It's slightly warmer than earth, slightly bigger and denser, and has plenty of carbon and plant life and liquid water oceans.
What the players don't know is that this planet is home to native intelligent life. They look like large metallic purple/pink salamanders with proportionally longer arms and the ability to briefly stand on their hind legs.
They haven't progressed much past the tribal stage but many tribes are large and they have pretty advanced metalworking and extraction methods. This comes from the fact that, although they started underwater, they didn't need fire because they evolved to be able to generate intense heat (just go with me on this) and can work with most metals that have a melting point accessible to Iron Age metalworkers. (Using tools they developed to enhance these heat generation techniques but the key thing is that they do not use fire all that often. (It's dangerous and scary and they usually don't need it.)
Most of this amphibious species lives in hollowed out "tree-coral" in the oceans of the planet, but they have begun to spread many of their homes onto land. They are also quite shy, the jungle is dark under the canopy, especially at night, they can see fine but the players can't. They're homes do not look from the outside like homes, they look like part of the wilderness (to the players anyway) and they do not speak with vowels and consonants. They speak with an entirely tonal language beyond the range of human hearing.
The R&D people who settled the planet know about this intelligent species and are keeping it quiet because the species has some brilliant medical research value. Not because it's alien, other aliens have been discovered, but something unique to them.
The players are on the planet to trade with them for their technology. The basic outline of the plot is that they go to the planet to trade, they explore the planet a bit and discover this species is intelligent, then it's up to them whether to overthrow the establishment or to complete their mission regardless.
What I am trying to figure out is what I can do to subtly demonstrate that these creatures are intelligent without completely giving the game away. What elements can I add to the world that my players will stumble across and begin to question? I can demonstrate intelligence unequivocally but that's not what I want to do. What would they discover in the woods that might make them question the sapience of native life?