Does your city currently operate its own system or do they use a county/regional/state system?
If the city operates under its own FCC license(s) you can search FCC applications for any new licenses or modifications to existing licenses, but there is no guarantee that they would do anything with the FCC if no new frequencies or transmitter locations were required.
Do you know any city employees or elected/appointed officials? Ask them if they know when it will happen.
You can search your city council agendas and minutes for equipment purchases or contracts. That might give you an idea of when it will happen.
If they are currently analog all you need to do is keep listening in analog mode without PL/DCS decode, PL/DCS search or auto-detecting digital modes. If they keep the same frequencies you will know when they switch to digital if you start hearing noise instead of normal audio.