It's hard to tell w/out actually testing them... but if one or more goes bad, there is a drop in perfomance across the rpm band. If they are clogged, it's harder to tell by testing, but the results are the same.
You really should have them profesionally cleaned every 30k-no more than 50k. I took out mine last year to have done and they cleaned them, put in new filter screens, new O-rings, and a couple new caps, then flow tested them to make sure they are in good working order... It was less than a $100.
I myself don't hear alot of Mustang injectors going "bad"... they used a Bosch on the 5.0's and I'm assuming it would be the same for the 4.6's.