The degrade happens over the space of 70,000 miles so it's hard to notice. Mine weren't leaking either, but when I took them off, I could move the piston up and down by hand. You might look for diving during sudden braking, noises, other suspension components starting to wear rapidly, uneven wearing of tires.
After 50,000 miles design life, the original shocks owe you nothing. If you do buy replacements, inspect the old ones during shock replacement DIY and leave the old ones on for a year if they are good. Expect after 72,000 mi the shock will probably move up and down in your finger tips, completely worn out.