To make NR work, it should be guaranteed that both UE and NW support NR capability. That's is, NW should know whether the UE support NR capability and UE should know whether the NW support NR capability.

There is a well known mechanism for NW to figure out whether the UE is capable of NR or not. It is through UE capability enquiry and information mechanism.

Then, How can UE figure out whether the NW support NR or not, especially for NSA?

UE can figure it out by checking a specific SIB2 (LTE SIB2) IE called upperLayerIndication. I know... it doesn't sound clear what exactly it mean. Practically it mean that the NW support ENDC capability if LTE SIB2 set upperLayerIndication-r15 to true. It implies that there is a NR cell colocated to the LTE Cell. The IE : upperLayerIndication would indicate some other things (e.g, other than the existence of colocated NR cell) in future release, but at least in ENDC environment this IE indicates the existance of colocated NR Cell.

Following is an example of SIB2 indicating 5G (NSA) supportability.

  message c1: systemInformation: {

    criticalExtensions systemInformation-r8: {

      sib-TypeAndInfo {

        sib2: {

          .....

          plmn-InfoList-r15 {

            {

              upperLayerIndication-r15 true

            }

          }

        },

For further details, refer to R2-1801529.