That isn't true at all. Most US networks, and may others globally support VoLTE. The difference in sound quality is remarkable, to the point of being unnerving at first.
You can tell if your handset and network are enabled by making a call while on LTE; if your network indicator drops LTE when you make the call, it's not supported and enabled. Most carriers have a help page with a list of supported devices and how to enable it. I don't know if there's any good indicators that you have an end-to-end enabled call, unless you can tell from the sound quality.