Get pictures of them and see if they still have the thin little groove lines in them and if they still have the wear indicators (they are little hole drilled into the tire face every so often). If they are no longer there I'd pass.
another factor to consider is the burnout....did he roll through the water box then spin em for a second, or did he have a line lock (or not) and do 2nd gear standing 5/6 second burn out to really heat them up
i think that would be something to consider too. best bet is to look at them like paul said...
my et streets lasted less than 50 passes. but i alwasy believe in a good burnout before each pass on them. 2nd gear burnout held until i can see smoke.