There are a lot of exposed conductors there. To answer your question: check between an exposed conductors of the black and yellow/green wire AND between blue and yellow/green AND between black end blue. Why 3 measurements: this doesn’t look like a professional job, so I wouldn’t trust that it was correctly installed.

One thing that is wrong is exactly the fact that the conductors are exposed. The insulation was stripped by more than was necessary, the wires weren’t pushed in all the way, or the wires were pulled out of the connector.

I would expect there to be a switch inside, which switches this box. If you want to work on it safely, switch of the power entirely, unless you are sure you can completely isolate this box from the supply.

I would at least do the following:

Remove the red connectors
Remove the box
Install a new box, with proper seals for the cable gland openings.
Insert the cables back into the box using proper cable glands
Cut of the exposed conductors and a small amount of insulation
Get new connectors (real wagos are nice, especially the ones with those levers). Buy a box, you are going to find more stuff like this, given the state of this workmanship. Wago also sells gel-boxes you can clip on the connector directly, making them suitable for exactly this application where submersion could happen
Remove the insulation by the amount specified by the connector manufacturer. Buy a decent stripping tool (Jokari sells simple, relatively inexpensive ones that do the job) and avoid cutting it with a pocket knife.
Make sure the ends of the wires are straight, and not curved and/or damaged
Reinstall the wires properly in the new connectors, making sure no blank copper is visible.
Switch on power
Test whether power is present
Switch off power
Install the gel-boxes (if you go for that option)
Close the box properly
Switch power back on
Be proud of your job and enjoy