If a caster is currently concentrating on a spell whose effects are not on the caster, can you cast Dispel Magic on the caster to end the spell?
No, you cannot get rid of a spell effect by targeting the caster with dispel magic
Dispel magic says:
Choose one creature, object, or magical effect within range. Any spell [...] on the target ends.
A person concentrating on a spell is not considered to be a target of that spell. To have a spell "on" you means to be under the effects of that spell. When you are concentrating on a spell (that doesn't target you) you are not under any kind of spell effect. The magical effects of the spell are entirely on the target(s) of your spell whether that be on another person (like bless) or a spell effect (like wall of force).
As such, casting dispel magic on the caster who is only concentrating on the spell would do nothing to help end that spell.
Jeremy Crawford has agreed with this interpretation in this ruling which covers the exact premise of this question:
Dispel magic is cast on a creature, an object, or another phenomenon that is under the effect of a spell. You don't cast it on that spell's caster. To dispel a spell like banishment, you'd have to somehow cast dispel magic on the banished target.
In this case, the spell banishment is being cast on a creature. Banishment requires concentration, but Crawford explicitly says that casting dispel magic on the concentrating caster will do nothing to end the effects of banishment on the banished creature. He says you would have to somehow target the banished creature themselves.
Official ruling in a very related case
In fact, even if the caster is concentrating on a spell this is affecting themselves that also has other targets, casting dispel magic on the caster will only remove the effect on them and not the other targets (and will not break concentration). An example of this can be found in the Sage Advice Compendium
If dispel magic targets the magical effect from bless cast by a cleric, does it remove the effect on all the targets? Dispel magic ends a spell on one target. It doesn’t end the same spell on other targets.
No matter how you look at it, casting dispel magic on a caster does not affect any spells not on that caster.
By RAW, I would argue 'no'.
The description of the Dispel Magic spell says:
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. [...]
The text of Dispel Magic says that it ends a spell on the target. Since the spell effect is not on the caster, it would not end the spell that the caster is concentrating on.
Similarly, concentration spells do not end if the target moves out of range etc. This would imply a RAI support for my answer as well.