The best way I found to make sure that I wasn't wasting my time was to use the bluetooth OBD sensor that I already had
Get any generic $15 bluetooth sensor. Plug it in, Test car using the Torque app. You can test for, look at and cancel check engine lights, but you can also see the status of the various long term cycles. Some will be grey, that means you don't have/need it. The others will be red, until enough miles to trigger the cycle, then they're green. Once all of the live checks are green, you can go get tested. They're basically giving you the exact same info the smog testing sight looks at.