There are generally three situations which get you a Test Only directive. One is if your car almost didn't pass on the last test. Second is if your car (model/year) is selected at random by computer to be sent to a Test Only station. And third if your car (model/year) has been identified as a heavy polluter.

You can take your car to a regular Smog Inspection Station and ask for a pre-test. You will pay but the results won't be transmitted to the DMV. If there are any issues, you can have them addressed. If everything is okay you can then go to the Test Only Station. I wouldn't spend any money on parts, additives, etc. until you confirm that it will/will not pass.

Good luck.