STPP<--->TSP
A good indicator of the status of stored STPP is whether or not the powder flows easily. If it does, it's probably NOT turned into STP. If it's as hard as a brick, it's probably turned.
The laws of physics stipulate that entropy is always increasing.
STPP is a higher energy molecule, and of lower entropy than TSP. So there is a tendency for it to degrade into the lower energy molecule. But I don't think it's as much of an issue for STPP that is properly stored as some here would have us believe.
Still planning on doing some testing with well water to see if my long term stored STPP is still active.
Also, a simple mnemonic to help remember which is which: The "T" stands for "Tri". The power of STPP is that it's got three (poly) phosphate groups in one molecule, hence "Tri Poly Phosphate". TSP has only one phosphate group, but three sodium atoms for every phosphate group. Thus, "Tri Sodium Phosphate".