Now that we looked at suspending and resuming, let's see how a coroutine gets created in the first place.
You cannot tell from a signature alone whether a function is implemented as a coroutine or not (i.e. whether it has any co_ statements in its body). An external library could flip a function implementation between coroutine / not coroutine without breaking API or ABI. The code to invoke it on the caller side is the same, and you get back an instance of the return type in both cases.
alcinnz