How to tell if drive train is original? ( 2004 legacy wagon )

[nevrknew]

So is there any easy way to tell if a car still has its original engine/transmission? 

Story:

A 2004 Subaru legacy 35th edition with 153k miles purchased for 2k. The car has had every body panel except the roof and trunk replaced with ones that are still rather dented, this is because carfax claims it was declared a total loss once and had "severe damage" twice along with 4 salvage or rebuilt title issues. (before the one it just got). Carfax also claims it got a new head & gasket at only 53k miles in 2009. 

The car was impounded after racking up 1k in storage fees at some storage place over a year ago, it was then sold for $650 at auction to a super shady jdm import guy who drove it without plates or anything but a definitively sketchy inspection for over a year. That sums up my knowledge before I ever saw it cracked windshield and all.

Although a cosmetic wreck the only meaningful mechanical issues are:

1. It is suspiciously low to the ground (who lowers an AWD)
2. The ac blows ice for ~20 mins and then cycles hot for 20 more and then repeats this loop

Other mysteries are who the some labels on the transmission that look state the model # of the trans as well as what looks like a stock #. These would make sense if they didnt look oddly new and like a hobbyist label maker printed them. These labels lead me  to believe it was a used unit purchased  from a junk yard or similar used parts seller.

NOTE: None of this really matters since it is only needed till mid to late September when a Toyota lite-ace from japan should arrive with only 16k miles. Thus my asking is mostly out of curiosity.

Thank you.