Principles for premature failure analysis of PCB tracks and vias
First response to what you write is:
One PCB is not a statistical sample size.

If you want any real information about failures in a product you design you need to test at the least a dozen, though most specifications require a hundred or more.

The same goes for your base of comparison: You need to test at least as many fresh PCBs to establish a baseline.

Of course if you're just getting your head around some design principles just doing one or two may be fine to get the coarse errors out, but just one or two is not a basis to guarantee anything for real-world use of a production series.

That said, if you have reason to be worried about failures at high temperature, just flopping the board through thermal cycles will not guarantee anything either.

Chips, transistors and currents create heat as well, so you need to establish whether the board will work when in operation at those thermal cycles as well as when it's powered off (if that is a remotely likely use case, at least).

For example a trace may not fail on your board when cycled from 10degrees C to 100degrees C and back a thousand times, but if the trace is carrying anything more than a simple (very) low current DC signal it may heat up for several degrees, or even a dozen or more. This influences your thermal cycle performance of that whole signal path.



All that said I mostly agree with @Master.

Any mechanical stress the real world might throw at it needs to be included in your test.

And further a more than x-percent (10% for example) change in resistance over a good number of cycles is the best hint something is going wrong. Your exact acceptance criteria will depend largely on your design and requirements.



Changes in Capacitance and Inductance are interesting, but only really if you are very strongly dependent on them, such as in high frequency signal paths, matched impedances and matched pairs. In which case you already know what your traces are designed for and what signal to use and measure to see if they have modified more than your design tolerance. 

Mechanical stresses on the board, such as through imbalanced expansion and shrinkage of layers may cause micro fractures or increases in distances between planes and traces, so a good thing to do is run all your tests again while bending the board mildly in several directions all the way at the end, to increase the likelihood you find them.

Of course you're free to figure out even better plans to find such.  I could envision some test plans including high voltage capable current sources while vibrating or bending the board. Sparking in micro cracks could be detectable in a dark environment, or afterwards by carbonation around them.