How can past duration of existing subterrannean termite infestation be determined?

Short answer: diligent use of monitoring stations in the area. 

Creative answer: If you still have some of the termites in question,
(presumably dead) you can always get an entomologist to get them
tested.

But results of long term field studies have shown that newly invading
termites will use existing galleries in the soil left by early
colonies. If that is the case, you might to look at existing
subterrannean termite infestations.

://www.google.ca/search?q=cache:xaASwxIGZ_UJ:www.bobgunn.com.au/Sentricon/Files/Evidence%2520Supporting%2520the%2520Use%2520of%2520Termite%2520Baiting%2520Systems.pdf+past+duration+of+existing+subterranean+termite+infestation&hl=en

Extra reading:

Here is an interesting article

http://www.pestcontrolmag.com/pestcontrol/article/articleDetail.jsp?id=77395