Having doubts about the symmetry of the detail, you can use the tool Symmetry check.

The program will display in the results Asymmetric walls – if they occur, which proves it, that, however, the 3D model is not fully symmetrical.

There can be two types of asymmetric walls: cutting i not cutting. Cutting to you, which is intersected by the plane indicated as symmetry, a not cutting lie outside this plane (these are often parallel or perpendicular walls).

The examples below.

Application: this part is not perfectly symmetrical. And because it is an import, then we cannot check the genesis. however, one can assume, that e.g., some of the walls were created by surface filling, which does not guarantee perfect symmetry.

Solution.

I cut half of the part and make it mirror with merging objects. Re-checking dispels doubts – Hello it is already perfectly symmetrical. These demonstrated asymmetric walls had micro deviations, so you could easily do this.