It's very difficult to tell when the stock struts are worn out. This is because they degrade very gradually, so you stay accustomed to their performance as they degrade. Plus, even when they're no longer responding as crisply as they did when new, they aren't an obvious problem. When they fail the old-fashioned "bump test" (push down on a fender and see if it rebounds more than once), they are long, long overdue for replacement.
In my own case, I never thought to myself, "Gee, the struts aren't responding very well". What I felt (on my '91 with 51K miles including ~60 track events and over 8K actual track miles) was that the car just didn't feel like it was handling as crisply and precisely as it did when it was new. There was no one thing I could put my finger on, but it didn't feel like it was on rails the way it did when it was new.
I bought Bilsteins. The Bilsteins have a very nice range of action, in which they are very responsive on the track, but manage to soften up on the street so they don't compromise ride quality. The OEM struts are actually pretty good for the exact same reason. I decided on the Bilsteins because I expect them to last longer than OEM. They also offer the lower perch to those who wish to take advantage of this feature; I left mine at the stock ride height.