You test your urine for ketones using ketone test strips.  Different test strips have different instructions.  Be sure to follow the instructions for the test strips you have.  Usually, you urinate in a clean cup.  Then you dip the test strip in the cup or use a dropper to place a sample on the test strip.  Wait for the amount of time shown on the instructions.  Then compare the color or the + signs on the strip to those on the packaging.

Your doctor can tell you and your parents what amounts of ketones are dangerous for you.  Usually, if you have zero or trace amounts of ketones, you're in the clear.  If you have moderate or high amounts, your parents will call the doctor right away. 

If you have moderate or high ketones, you'll probably take extra insulin (how much is up to your doctor).  You should also drink plenty of water and avoid exercise.  In extreme cases, you could go to the hospital. (but that's rare).