I'm trying to track down the title of a short story I read at least twenty years ago, probably nearer 30 and may not have been new at the time. Almost certainly in an anthology.
The starting premise is a human crew stranded in space and with some sort of time constraint - it may have been specific to them (limited food or air), or an external issue (limited time to complete a mission). They find what appears to be an abandoned ship which acts as a zoo - a number of different alien species are on board, each in their own enclosure - but no crew.
The determine that one of the species on board is actually the crew, acting like unintelligent animals to throw the humans off the track. The humans have some sort of mind probe that can test for intelligence - but only one species at a time, and it will take too long to run through them all.
Someone - quite possibly not the 'obvious' character - works out that the crew are actually symbiotes. There is a small, intelligent species (possibly looking like a helmet) paired with a larger species (possibly humanoid, bear-like, or ape-like) that actually pilot the ship. Once the actual crew is identified, they appear to reach an amicable arrangement fairly easily.
A couple of other elements:
When the control room is found, they activate a control for a 'jet boost'. The switch was on a light contact, which they used to rule out a high-g species.
Exploring the ship, they found living quarters with bunks and some small shelves, which turned out to be bunks for the two symbiotic aliens.
There was an attempt to use the atmosphere as a discriminator, but the ship had a device that would reproduce an atmosphere it was exposed to; once the humans had gone, they would 'open the door of their cell a crack', release some of the atmosphere, and wait until it was made dominant by this device.