In 4G LTE, each cell broadcasts a parameter called upperLayerIndication-r15, which can be used by the network operator to indicate that the 5G NR service is available in that cell (see for example here). This parameter is part of the system information, and more specifically appears in SIB 2. Mobile terminals in idle mode read this parameter, along with the rest of system information.
I have not found this parameter in Android's API documentation. Is it available to applications, perhaps with a different name?