How to know if MacBook Pro with LG 5K Monitor Displaying at Correct Resolution
Under the display tab in the screenshot you posted there should be a "Resolution:" option. Click on the "Scaled" option to the right of it and it should show several resolutions, with the one you are currently using highlighted. If the resolution you want is not listed trying hold option and clicking "Scaled" for more options.

Alternatively, if you are comfortable with the command line, the follow command should list the resolutions used on all active monitors:

system_profiler SPDisplaysDataType | grep Resolution