When a pirate wanted to mark the exact location of his treasure on a map, he marked it with a red X. Photoshop has a similar function.
When you need to mark an exact spot on your Photoshop or PSE document, just set the Caps Lock. Doing this will change your cursor into a cross hairs (like a +) so that exact precision is easier. This works especially well when using the Brush tool, the Eraser tool, and the Clone Stamp tool. Zoom in by pressing Ctrl and the + key (Mac: Cmd and the + key) to get even closer to your subject!