Let’s have an image with a size of e.g. 1000×1000 px. The image is transparent, but only but one pixel is not.
How can I find this pixel where it is in the picture by using Photoshop?
Can I see from somewhere how many (non-transparent) pixels my image contains (by using Photoshop).
You can Ctrl / Cmd + click the Layer thumbnail to select all non-transparent pixels and read the number of pixels in the Histogram panel:
The downside is, that the pixels must have 50% opacity or more to be able to be selected this way. Might need to make sure that is the case before doing the count.