While waiting for a delayed outbound flight how can I guess where's the inbound plane? This summer in Mykonos our flight back to FCO was delayed and nobody at the airport could communicate reliable estimates on when it would be.

In that case I managed to find the FCO-JMK inbound flight on FR24 and saw it left FCO with 2 hours delay, so our own would have been at least that.

In bigger airports with more connections you might not be able to guess which airplane will become yours.

I guess the registration number of the plane assigned to my flight will be available some time (maybe a few hours?) before as per the flight plan.

Is there a reliable public accessible source for that?

When I have the assigned RegNum I can use plane finder, FR24 or whatever to see if it's on its way and have a rough estimate of when it will land and then be available for my flight.

Apologies for my English.

Thank you
It's difficult to find out what the inbound aircraft is, unless you are at a small airport and/or the airline you are flying on is only operating a few routes from the airport. When you are at a big airport and/or the airline is operating many routes there, you have no way of telling where your aircraft is. For smaller airports, you can effectively find the inbound aircraft by using the appropriate filters in FR24. 

Note that in some cases the aircraft planned for a flight can be changed last minute. This typically happens when aircraft are significantly delayed inbound to their hub / main base and the airline decides to minimise the outbound delay costs by assiging a different aircraft.