Source Aircraft [which inbound aircraft will serve your outbound flight?]
 

How can you tell what inbound flight is scheduled for a route?

For example, if I know that AA flight 998 is used for flight 999, then I can sometimes tell that 999 will be delayed if 998 is.

Also, if I want to do a mileage run and I know that 998 flies from A to B and then turns into 999 flying from B to A, I can (somewhat) safely book this as a return.

I realize these are not absolutes, as it's possible in case of a serious delay they could roll out another aircraft.

Flightaware.com has a feature where it'll show you the inbound aircraft from the page of the current flight information. Other than that - there's very little rhyme or reason and a flight can change aircraft many times in the hours prior to departure.

The only surefire way is to pick a route that the aircraft does a definite out and back to an outstation. Other than that - it's way too fluid to make any meaningful plans.

AA's website (and often app) has a similar feature. Just pull up the status for a flight that hasn't departed yet and there's a "Incoming Flight Information" link.

Also, 998 may not always turn 999. It's very rare (unless, as mentioned, it's a small outstation) for an routing to consistently be operated by the same aircraft.

On the other hand, if the turn-around destination doesn't have a lot of flights arrive at the same time, and your inbound arrives less than an hour before the outbound and is the same type of aircraft, it's a good bet your plane is doing a turn. I've done MRs like this lots of times.

Once I've figured out the departure gate, I sometimes use Flightstats to search by airport...for large airports you can sort arrivals by terminal and gate...to find the inbound. Benefit is that you can then track the entire record for that flight to get a sense of whether it's likely to be on time or not.