Only tell-tale signs, like importing modules that only exist on the one, or using constructs like print-as-statement or print-as-function. Nothing guaranteed to let you know at a glance.

And of course, people can write code that runs on both. But I think that's probably easier to spot (or trick yourself into thinking you spotted it?), because the top of the file would be full of things like `from __future__ import print_function`.