1998 India/Pakistan was a rough year; but every nuclear power has tested their bombs to make sure they work.
There are detection systems that were able to detect sub kiloton explosions in NK from across the world; with triangulation that's quite precise. We would know if Israel has tested.
If they havent tested, they really dont have anything.
Now lets assume they do in fact have some; perhaps smuggled or sourced plans from some other nuclear power.
Why are they allowed to? Well what are you going to do about it? They'll nuke you to keep them.