Taking the grills off - and especially getting them neatly back on - is tedious work, and the attaching clips may break in the process. Bring a small bright flashlight and you can visually inspect the drivers with the grills on, see attached picture. However, it will only show if the surrounds already have holes in them. Long before that, they become sticky and fragile, which you cannot see from the outside.
The only drivers susceptible to foam rot in the Pentas are the midranges, the woofers (and the tweeter) will be fine. The midranges are the four smaller drivers stacked in the middle.
However, since the midranges move very little during normal play, even the sticky surrounds may still last a long time before coming apart. So the visual inspection might be enough, and the seller may be pi**ed if you try them with your finger and they break 
