The only easy visible difference between the 2 core is that high oil
pressure sender on the 2.1L
http://www.benplace.com/engine/sender4.jpg
http://www.benplace.com/oil_pressure_sender_overview_location.htm

The ONLY other way is to open the engine and look at the stoke of the
pistons... the crank (piston stoke) is the main difference between the 2 engines

The short story is: i never seen a rod go through a case on a 1.9, but some
on 2.1.... VW widen the 1.9L rod big end for the 2.1 (not my theory but I
approve) so some say that it as been weaken! who knows...

But here what i can tell you, i have seen very weak crank and rod bearing on
2.1, never on a 1.9

My 7 CAD cents

Cheers, Ben
http://www.benplace.com/