How can I tell if the aicraft for my departing flight is at the departure airport? [duplicate]

I use Flight aware for this. Here's an image of a flight that leaves in an hour or so:

Both Where is my plane now? and Track inbound plane (which I've circled) lead to a page just like this, but for the incoming flight. It's a great way to know how true Air Canada's favourite excuse, "late arrival of the inbound flight", really is.

Unaware of the quicker approach in Flightaware, I had used a workaround making use of Flightradar24 previously:

1. Search for your flight or use the direct link https://www.flightradar24.com/flight/ba202 where BA202 is your flight number. You get a results page with the flights of the last and next week.

2. Click the aircraft registration code (the second item, in brackets, in the aircraft column) and you will see the previous and current flight of your plane. Since your plane in this case has not been specified yet, I am providing the previous' days aircraft for illustration: G-VIIN, i.e. https://www.flightradar24.com/reg/g-viin is your link.

3. By clicking the aircraft symbol to the right of the most recent flight, you can figure out where your equipment is currently located.

There's yet 7 hours until the departure of that flight. Presumably the airlines's operations department has not yet filed a flight plan, which tracking websites would be using to match a particular aircraft to the future flight.

However, I note that the flight you linked to (in a comment on Kate's answer) is BA 202, which goes from an overseas airport to the operator's hub in London. Thus there's a really good chance that it will be operated by an aircraft that has just completed an outbound flight in the other direction.

Searching on BA's website (for flights a week from now such as not to confuse it) we see that flights from London arrive in Boston on Fridays at 13:30, 19:35, 20:50, and 22:20, and leave at 07:50, 19:20, 21:35, and 22:40.

Evidently one aircraft stays in Boston overnight, so assuming they depart in the same order they arrive in, you want the one arriving at 19:35, which is BA 203. (As a check on this conclusion, the flight numbers are next to each other, which is usually the case for outbound/inbound flight pairs).

Flightradar24 shows BA 203 as a Boeing 777-200, registration G-VIIC, which is currently almost halfway between Ireland and Newfoundland (and expected to arrive about 20 minutes early).