It's a very simple question - is there a way to tell that a given signal cannot carry digital information. I don't have to think very hard to realise that obviously if something is a signal, carrying information, then it can carry digital information, of at least one bit. It's almost trivial, once you actually think about it.
If there is no way to tell whether a signal does not carry digital information, then it necessarily follows that it's not possible to distinguish digital signals by their content. This isn't actually a particularly difficult concept to grasp. A => B =>C.
Any analog signal will have variations, because if it doesn't it isn't a signal. Any variation is sufficient to carry digital data.
When local telephone lines were converted to use DSL, this was described as a switch to digital. Now, before that, modems were used to send data in a different - less efficient and reliable - format. Was the data sent via 9600 Baud modems any less digital than that sent by the DSL? Of course not. "Digital", to communications engineers, is just a matter of convention.