I agree with many others, 1, 3, 5 look most suspicious (assuming numbers precede pictures)

Others seem to have heavy upscaling but it doesn't tend to become too patterned. In 1, 3, 5 grass tends to become something like a fractal rather than grass, plus reflections are sometimes off (that is why 2 is a bit suspicious because of a small tree reflection, but maybe it's just hard to see the original tree), and the branches tend to grow from multiple trees at the same time