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.