While waiting for a delayed outbound flight how can I guess where's the inbound plane?
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.