An "amorphous blackbody" source, from anything reasonably distant, just looks like a smaller star. There are a bunch of if's and unknowns involved, but it could be maybe possible to detect a Dyson sphere. This[1] article talks about it. The upshot is that it is nowhere near blindingly obvious, and we don't currently have the ability to differentiate between a natural or artificial infrared source.

https://en.wikipedia.org/wiki/Dyson_sphere#Search_for_extra-...