How to determine if a package is ROS1 or ROS2?

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I don't believe there is a canonical way to determine this.

You'd have to rely on circumstantial evidence, as you already describe.

ROS 2 packages wouldn't/shouldn't depend on anything to do with Catkin, but there are exceptions, like the ros1_bridge (here fi). That's not a common thing to do though, but enough to be able to say: you can't strictly rely on this.

That would leave implementing heuristics, which you've already done I believe.

What I am hoping to do is look at a directory, determine if it is a ROS1 or a ROS2 package, and then use the appropriate message parsing logic depending on the package type.

If you're mostly interested in messages, I would check the package.xml for dependencies on message_runtime, message_generation, rosidl_default_generators and rosidl_default_runtime. The former two: good chance it's a ROS 1 package with messages/services. Latter two: ROS 2. In CMakeLists.txt you'd expect to see rosidl_generate_interfaces(..) for ROS 2, and generate_messages(..) for ROS 1.