Put a jack under the axle under the frame on one side and jack up the trailer. Note how much the wheel/tire moves relative to the trailer as you lift it off the ground.
If it comes off as you jack up the trailer just a little the axle is dead!
If you have 2 - 4" of relative movement then it is probably OK.
My 16' 1986 was the same as having a solid axle and all of the shock absorbing was in the tires
Also as you walk around and jump un p and down you should feel the trailer bounce, not as much as a leaf spring axle, however since the rubber does damp out some of the oscillations.
Riding low is also a sign of a dead axle.
If you get the number off the tag on the trailer and call AL-KO they can give you the original specifications for it and also sell you a replacement.
I went with the Flexiride axle since I did not know what I wanted for a finished ride height and theirs is adjustable.