Worried about the security of your Android? You probably should be. But the guys that discovered the Stagefright vulnerability have now released an app that that will tell you if your phone is at risk.
While Google has already created a fix — with some fanfare — currently only Nexus devices have been updated. If you use any other Android device, even a brand new one, you're likely still at risk. You can use the Stagefright Detector App to find out.
The app can't fix the problem for you, but it will let you know if you have a gaping security void in your hand. And if you know that, you can take some precautions: namely, not opening video MMS from strangers, and disabling "Auto Retrieve SMS" if you use Hangouts as your text app.
Until you phone is definitely patched, that's the best you can do.