Anders, if you do not have a light meter, then I would suggest setting up in a room, put a middle tone subject about 1.5 meter away with the background at about 2 meters, to check TTL (& S-TTL) take one strobe at a time and take pictures at 1 stop interval, the exposure should remain fairly constant (do give the strobe time to recycle properly), swap to the other strobe and do the test, at this point, if both strobe are in good shape, then result should be faily similar (might want to comape histograms), if one strobe is defective (or wrongly set up) then exposure will be all over the place. for good measure, if you have a Nikon flash, do the test with it as well so you have a reference set of images, Nikon's TTL is notoriously precise.
Just an idea I had, there is a magnet switch on the Z240 that manages the pre flash, are they both on the same position?