Foestats does have a history which shows points earned and battles fought daily on each person's page, and even if the updates aren't actually quite daily, over the long haul everything will even out.
If you see from the graph that the last time someone's total increased was a month ago, then it is pretty safe to say that they haven't played in a month.
Maybe they logged in during that time, but if they didn't do anything meaningful what's the difference?