If you want to go to a pixel you move your mouse/pointer, looking at the pointer coordinates at the bottom of the window. You can of course zoom in to make it easier to hit a given pixel.
If you want to "mark the spot" there are several ways:
- You just create two guides (vertical and horizontal) that intersect on the pixel.
- You use the Measure tool. Move to the pixel, click, then depress Alt+Ctrl and click again. This creates two guides that intersect on the pixel
You can also create samples points. These are created like guides, by dragging from a ruler, but also depressing Ctrl at the same time. You can later move/delete them using the Color picker tool. When you have sample points, you can have their values permanently displayed (as RGB, HSV, LCH...) in the Sample Points dialog:
These values will even change while tools are used.