If I want to see where a particular aircraft (airline) has been prior to the flight I’m tracking, how do I find this? I know the current flight number, but have no way of knowing where’s he’s been. Just interested and curious. Thanks. 
That would be difficult. You would have to know the flight number it was when it landed where you are, ie USA 121 PHL-CMH is USA 313 CMH-ORD (made up flight numbers). If you didn’t know it was USA 121 when it landed I don’t think you can do what you want. Also because airlines fly via flight numbers and not registration numbers, registration number N12345 for example are aircraft specific.
Sorry I can’t be more help.
We’ve been through this before, and I’m surprised you-know-who hasn’t pointed this out in his own inimitable fashion 
Anyway the steps would be:
- determine which gate flight A departed from
- look in the airport’s FIDS for the previous arrival at that same gate - flight B
- use flightaware to find the origin of flight B
- use an ACARS site to get the registration for flight B - tail number N
- use an ACARS site to trace previous records for tail number N.
Of course the above could break down at any point, the main one being if there was an equipment swap for whatever reason at gate 1).
Hope this helps. Yeah, right !