You know when each individual explosion is done. You know how many explosions there will be. Just make whatever is in control of your explosions (random event handler?) trigger an event saying the explosions are finished. If you don't have a separate object controlling your explosions, make one.
A global event handler will likely be very useful to you. Any object should be able to push events to the handler. And any object should be able to be extended to handle events.
See these questions for more details:
Best way to manage in-game events?Best way to manage in-game events?
What role does developing with events have in games?What role does developing with events have in games?
What should I be considering when designing an Event Manager System?What should I be considering when designing an Event Manager System?
