Think about this: you were blindfolded, bundled into the trunk of a car, and then unmasked at the other end of a long journey to be presented with a blinking prompt. How would you determine what sort of system you were on?
Your immediate instinct might be to assume you've been placed in front of a Linux box, but that's not a certainty. While it's true that Linux dominates the server's space, just because the instance has a black screen, white text, and a login prompt, doesn't mean that you've been placed in front of our friendly penguin OS.
It could be Linux, a BSD system, a Solaris system, or one of the many Unix derivatives from the nineties.
Assuming you've been given the credentials to log in, do so.