It's definitely worth using distilled water when mixing up a new batch of Star San. Then once it goes somewhat cloudy it's time to pitch it.
But if you'd really like to know if it's good with some greater accuracy, then you'll have to invest in a cheap ($30) pH meter and test it. If it's below 3.5, then you're okay. Test strips aren't very accurate, so I'd advise against using them.
I'd advise that you invest in a better pH meter (around $80) and get a meter that can read in the hundredths (x.xx.) You could use that pH meter for testing the pH of your mashes as well as guaranteeing your Star San is still good.
I bought the cheap pH meter first and it only read into the tenths (x.x). It was only accurate to plus or minus .3, so if the meter was reading 3.5, then the pH could have been anywhere from 3.2 to 3.8. That's not accurate enough for our purposes. Money wasted but lesson learned.