Boessie, if you can see the twin TCAS antenna, it is a short nose. If you can see a bubble over where the antenna is, then it is a long nose.
Anything above 31200 and 41200 is long nose, although there are two different long noses depending on the radar fitted. RDR-1500 has the loooooooong nose fitted.
Edit... Having worked on 31406 I can confirm that it is indeed a long nose.
The only exception to the above serial numbers is 31007. The seventh 139 built was the long nose prototype.