A relatively low flat cloud base at 2000 feet off the ground would intercept the horizon at a distance of about 20 miles beyond the distance of the visible horizon itself. That's not taking into account atmospheric refraction, which extends that distance farther.
With foreshortening, atmospheric scattering, and with clouds typically being low contrast in the first place, I don't think it's possible to clearly see the cloud base at that distance.
For a typical contrail at 25000 feet altitude, the distance of the contrail's horizon beyond the ground horizon is about 72 miles. A contrail is clearer than a cloud layer, but with that distance, you won't be able to see it "meet" the horizon clearly under most conditions.
As for curvature, a 72-mile stretch of contrail only curves about half a degree. That's enough to be clearly visible if the contrail is highly foreshortened (i.e. if it's on a course passing close to directly overhead). It's going to be harder to see the smaller amount of curvature in e.g. a 20 mile stretch of cloud layer.
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