[tutfbhuf]

Yup. It's absolutely possible to determine whether you play versus a strength reduced/weak engine or a human. Humans make human plans and play according to human best practices, which you can find in chess literature. Engines, however, do not make long term plans and often play very inhuman (perfectly precise, unobvious tactical) moves, that often suprises even on grandmaster level.

[reidmcy]

I'm not sure if saying engines don't make long term plans is accurate, since we can set them to plan an arbitrary number of moves ahead. I think you mean that they don't have a theory of mind, and thus don't modulate their play based on their opponents.

[watwut]

No, he means abstract plans. Abstract plan is not exact sequence of moves, but rather general idea of where you want to be in far future.

It could be modulated based on opponent, but does not have to be.