A planet activates a midpoint if it's in conjunction, square, or opposition to the midpoint. To see if you have a planet activating your Jupiter/Pluto midpoint, check to see what degree of Capricorn it's at. Then look at your chart. Is there a planet at approximately that degree of Capricorn, Cancer, Aries, or Libra?
If yes, that planet activates your Jupiter/Pluto midpoint. If you have more than one planet in the right place, they all do. If you don't have any planets in the right place, you don't have a natal planet activating your Jupiter/Pluto midpoint, but you get that midpoint activated by transit whenever a planet transits that degree of Capricorn, Cancer, Aries, or Libra.
You can also have your midpoints activated by synastry. If another person's chart has a planet in conjunction, square, or opposition to one of your midpoints, and you have a relationship (romantic, familial, or platonic) with that other person, they activate that midpoint for you. Planets that activate each other's midpoints are one way people can be astrologically compatible: if you have a midpoint that isn't activated by any of your planets but is activated by another person's planet(s), they bring that part of your chart out in you.