You can tell by making a call. If “LTE” or “4G” remains visible in the notifications area during the call, and doesn’t drop to “3G” or “H+” then you’re using Voice over LTE.
(Voice over cellular/telephony network is 3G or 2G. Voice over LTE is voice carried over the 4G/LTE data network.)
BTW, if you have a mobile setting similar to “Enable VoLTE” or “Enable Voice over LTE” or “Enable 4G calling,” then the phone can use VoLTE.