Surface scratch between neighbor SMD components
How to fix:

Clean the two pads with solder-wick. Then cut a small piece of self adhesive Kapton tape (polyimide) and put it over the scratched surface but not over the pads. 

The tape will not melt while you solder and will prevent the solder from forming a bridge, so it works as a temporary solder resist.

Now carefully solder your components. Inspect your work using a microscope. If you're happy with the result remove the tape. You can paint a little bit of solder resist over the scratch if you want. 

If you don't have a microscope maybe it's time to invest into one. There are affordable USB microscopes. They aren't perfect while you do the work because the picture is delayed, but they are pretty good for inspection.

Regarding your question how to know if the pads should be connected or not: If you don't have the board files and the traces are not visible you can measure a non scratched board to find out.