Black holes tend to occur in large "clusters" alongside neutron stars about 1000 LY above and below the central plane of the galaxy. Go Up or Down and you should find plenty of them.
Ammonia worlds are, in my experience, not as common as Water Worlds, but they are more common than Earth-likes. Any perceived imbalance in favour of ELWs on player-submitted-data sites like EDSM is probably reflected by the fact that many hardcore ELW hunters only scan ELWs and don't even bother scanning any AWs or WWs they find.
For me, spotting AWs is as "easy" as spotting ELWs.
- On the System map, AWs are usually a mixture of white clouds with a brown-and-red mottled surface. They can look superficially similar to the white-and-brown HMCs. It helps that I have an old, slow, low-grade computer that shows planets as blurry coloured blobs when you first load up the map, and the AW has a very distinctive brown-red-white-striped blob.
- They always occur outside of the Goldilocks Zone, in areas of the system that are too cold for waterworlds. Even the AWs that have room-temperature surface conditions cannot form in the ELW Goldilocks Zone.
- They have a distinctive sound when zoomed in on the system map, it's a very quiet, subtle sound, lacking the heavy-metal distorted guitar you get on a HMC.
- Finally, the icon shown in the lower left quadrant. It is the same one used for Earthlikes, with oceans and islands. It has a prominent, clearly delineated small "island" at about the 2 o'clock position that the HMCs and Rocky-Ice worlds with similar-looking icons do not have.