How do I know when shocks/struts need replacing?

Struts wear gradually so you don't notice the degradation. The best test: push down hard on each corner of the car and let go. It should bounce once and settle. If it bounces 2-3 times, the strut is worn. Also look for oil leaking down the strut body. At 90k on a Camry you're in the window where they typically start going. Not urgent if the car rides OK, but worn struts increase stopping distance by 10-15% and reduce tire life.